Generally speaking I agree with Jake. I do think, though, that the topic is worth a little discussion.
I'm a big booster of plastic sailing ship kits. (That's probably a waste of time on my part, since the genre is just about dead, but I still think they have a great deal to recommend them.) Styrene is a wonderful material for lots of purposes. It also has some built-in limitations.
For one thing, the thickness of the "walls" of a hollow plastic casting are limited by the laws of physics. If they're too thin, they won't support themselves. If they're too thick, the styrene shrinks noticeably as it cools, resulting in the dreaded "sink marks" (among other things). That's why the bulwarks of the Revell Cutty Sark are way too thick for 1/96 scale. (Those bulwarks are made of sheet iron; in 1/96 scale they should, theoretically, be about the thickness of aluminum foil.) The bulwarks of the Revell Constitution, on the other hand, are far too thin. In the former case, the discrepancy is almost impossible to see, because the edges of the bulwark "plating" aren't visible. If I were doing a Revell Constitution, I'd look into the possibility of cementing some plastic strip around the insides of the gunports, to beef up the bulwarks where the edges are visible.
The wonderful old Japanese company Imai just about conquered that particular problem by using an odd form of styrene that, somehow or other, allowed castings of considerably greater thickness than is possible with regular styrene. Some of those Imai hull parts were really massive - and none that I ever saw suffered from shrinkage or sink marks. We lost a lot when Imai went out of business.
Another feature of styrene is its flexibility, which, of course, increases as its thickness is reduced. A styrene strip or rod less than 1/8" thick, or thereabouts, is extremely flexible. That does indeed put some obvious limits on the material's usefulness in sailing ship models, since lots of spars - especially in smaller-scale models - have to be smaller than that if they're to be to scale. (One feature of that excellent, 1/125 Imai Cutty Sark that I remember is that some of the upper spars were a little too thick. Even the geniuses at Imai couldn't get around every limitation of the medium.)
Back in the olde dayes of the fifties and sixties, the good folks at Revell made a gallant effort to mold the masts and yards of their sailing ship kits as close to scale size as they could. In the case of some of the smaller kits, that made the upper spars pretty brittle. If you bought a Revell Flying Cloud, there was a fair chance that one or more of the skysail yards would be broken before you opened the box. In the case of the big Cutty Sark, though, my recollection confirms Jake's observation. That particular ship's top hamper was sturdy enough that it could be reproduced in styrene at 1/96 scale without a major problem of flexibility.
One big caveat, though. It seems that in recent years Revell has been using a considerably cheaper form of styrene than it used to. The last Cutty Sark I started (and never finished) gave me a problem I'd never seen before - and haven't seen since. I used masking tape on the waterline, and when I pulled the tape off a layer of styrene came with it. The stuff had the consistency of mica. That was a long time ago - and I have the impression that the situation has gotten worse. Several folks here in the Forum have reported that the styrene in such kits as the reissued Kearsarge and Alabama is awful stuff - rubbery, riddled with flash, and frequently warped. If somebody told me the spars in a recently-purchased Cutty Sark kit were two or three times as flexible as the ones I remember, I'd have no trouble believing it.
Before anybody starts buying wood to replace the spars of a plastic sailing ship kit, a couple of other points need to be born in mind. In the first place, the choice of wood is crucial. Some woods are stiffer than styrene; others aren't. If you try to turn a piece of balsa or basswood down below 1/16" in diameter, it probably will fall apart before you're done. The dowels sold in American hardware stores and hobby shops these days seem to be made of one of two woods: oak or birch. Oak is far too coarse for most scale modeling purposes. Birch is better (as long as the grain of the dowel is straight), but when it's reduced below 1/16" or thereabouts it gets just about as flexible as styrene. Harder woods obviously don't bend as easily - but they do break, frequently at extremely awkward moments.
The rigging of a sailing ship is designed to make any flexibility in the spars almost irrelevant - at least in theory. In theory it shouldn't matter much whether a spar can bend or not; if the rigging is set up accurately, and uniformly tensioned, the spar won't bend. That's in theory. In practice, it's difficult to set up all the lines to the same tension - and if they aren't at the same tension, something starts to bend (or breaks). If the spars are a bit more rigid than the typical plastic kit part, that makes the rigging somewhat easier by increasing the margin of error when it comes to tensioning the lines. But wood spars don't come close to eliminating the problem. Replacing styrene spars with wood ones certainly is not guarantee of success.
For what little it's worth, here are pictures of three models that take three different approaches to the problem: http://www.hmsvictoryscalemodels.be/johntilleygallery.htm
The Bounty is based on the 1956 Revell kit. I replaced all the styrene spars - not because they were styrene, but because their dimensions and shapes were inaccurate. I made the replacements of a hardwood called degama (which I think is known in England as lancewood). It's quite hard and stiff, and extremely brittle. (The fore topmast snapped when I was almost finished with the model; I was able to fix it with CA adhesive and a metal pin.)
I also used degama for the spars of the Hancock. It's scratch built, on 1/128 scale (3/32" = 1'). That model required some extremely small-diameter spars. (Such things as the flagstaffs are well under 1/32" in diameter.) In most cases it took me at least two attempts on the Unimat to get them down that small, and I broke more than one of them before the model was done. (Incidentally, the hull planking, gun carriages, transom, and numerous other parts of that model are styrene. It's a wonderful scratch building material, if its benefits and limitations are born in mind.)
The Phantom is based on the now-discontinued resin-hull kit from Model Shipways. (MS currently sells a version based on the same plans with a pre-carved basswood hull.) Having run out of degama (and having no idea where to buy more), I made this one's spars out of cherry. It's nice stuff; I'll probably use it again. It has a nice, tight grain, turns well, and is almost as stiff as degama without being as brittle. Its biggest drawback, perhaps, is its color. Cherry is notorious for turning darker as it gets older. Within a few years I may well regret picking it.
If I were building a Revell Cutty Sark today I'd start by checking the consistency of the styrene. If it was really rubbery, I'd probably replace the spars with wood (or maybe, in some cases, brass rod). Otherwise, I'd keep the plastic spars. There are, however, some other plastic parts that I definitely would replace. Plastic belaying pins are no good. (In most of the big Revell kits I've seen in the past fifteen or twenty years, a high percentage of the belaying pins have been busted off before the lid comes off the box.) I'd regard the purchase of some aftermarket brass belaying pins as just about mandatory. Same goes for plastic eyebolts; they can break too easily. Several Forum members replace them with either aftermarket eyebolts or small cotter pins; I personally refuse to spend money on manufactured eyebolts, and bend my own from brass or copper wire. (It takes less than a minute per eyebolt.)
Styrene is great stuff, but it's not ideal for every component of a sailing ship model. I suppose the ideal sailing ship kit might have a styrene hull, hardwood spars, and fittings made from a mixture of styrene, brass, and britannia metal. Unfortunately it seems highly unlikely that we'll ever see such a product.
Well, this turned into a far longer post than I intended. I apologize - and plead guilty to using this Forum as a treatment for insomnia.