I build limited run kits mainly because for so many subjects, they are indeed the only game in town.
Also, they develop modeling skills in a way that a lot of mainstream kits don't, both the physical skills like being real delicate with the knife or file to get something JUST right, to the less tangible skills like thinking ahead a few steps to see problems and cook up solutions before you get to them.
I think the closest mainstream kits ever got to limited run was the old Matchbox kits. Some rarities in that line that you're not likely to see made in kit form again (unless Revell Germany sees reissuing something as profitable). I still love the old Matchbox kits, the shapes were usually dead on or very close to right, but they tended to be quite spartan otherwise so you could do practically anything with them from scratchbuilding experiments to make the perfectly accurate cockpit to letting your imagination go wild and making a totally hypothetical variant of the thing.
My first limited run kit was Pavla's 1/72 Siebel fh-104 Hallore
I'd never heard of the type before, but thought it was a really attractive aircraft. It was a rarity in real life too, only about 80 of them were built.
I've bought a couple of limited run kits since I've been here in the Czech Republic: An Aero 145 and a Rutan Voyager. both in 1/72 and both by A-Model.
I've also got a Classic Airframes 1/48 F-5A kit sitting in a box back home in Canada waiting for me to build one day.
I'd say limited run kits can be quite adictive especially when you have someone look at a build on your shelf and ask you all kinds of questions about it because its almost a totally alien subject to them. There's a certain rush to that experience