Ahh, my favorite subject... While I don't have any problems with mixed-media kits, I don't agree with the predelection of some folks to add a hundred bucks worth of after-market parts... I myself add a LOT of what is, technically, "after-market", because it didn't come in the box, but rather the Evergreen, Plastruct, and brass wire, tubing, and sheet bins, along with parts from the ol' "junkyard", and I can also cast, carve, and vacuform parts... I've also purchased some after-market resin stuff, but it's for making copies for multiple builds;I use them as "masters"... For instance, I bought an M-109 SP howitzer interior detail set. In and of itself, it violates my own set of rules about "cheating" in a build, but in this case, I considered it a necessary evil, since I have to not one, not two, but six M109s to build for a former Battery Commander's retirement party, and there's a deadline... ( I have to build the entire gun-line, with the M548's, the BC's Humvee, figures, and all the goodies that one finds around a gun battery's position..) So, it's a great time-saver in this instance (and I'll never do it again)...
On the other hand, I believe that some modelers have become so dependant on the stuff that they will never develop the skills necessary to do it any it other way, and why should they try? I learned to scratch-build simply because there was no alternative... The stuff didn't exist, or only existed in small quantities and for limited kits, so there was no other way to reach the level of detail that I wanted without putting knife to strip & sheet...
I also have to balance it with the "seen or not" question... Frankly, I don't do it if it's not visible after completion, and in my shadow box dioramas, if you can't see it from the viewing window, it ain't there. Some of this I learned the hard way, putting a lot of time working out the interior ribbing and wire-runs inside the bombbay of a B-17 and as soon as I installed the bombs that were required for the build, I discovered that NONE of it could be seen from ANY angle... Couldn't even see the freakin' BOMBS, with their cleverly applied graffiti, fer cryin' out loud... That was there and then end of THAT nonsense, lol... It was like, "well, there's two days of my life I'll never get back"...
It all boils down to "If you think you need it, you'll do it anyway, regardless of what I think"... You'll also likely blow a gasket when my 30 -year-old, Evergreen-ed, Plastruct-ed, and wire-ed, non-after-market parts- stuffed M-41 with 2.00-worth of extra materials blows away your new-release 79.00 kit with the 100.00-worth of photo-etched and resin parts all over it at a contest... (Ain't braggin'-just happens to be so, but that's another story.) I enjoy bein' able to answer the question of "Who's parts did you use there?" with, "Mine."
Overall, I enjoy doin' things "the hard way" (I don't feel it's hard at all) when it comes to detailing and super-detailing. Plus, there's a real, tangible benefit from "my" way of doing things with base materials and a few tools, not to mention that I saved a LOT of money by NOT buying AM stuff and putting that coin into buying the tools and equipment that allow me to make the parts... I don't care if an important but tiny, delicate part I'm trying install suddenly *pings* outta the tweezers into Oblivion... Many times, I won't even bother to look for it, it don't mean nothin'... I'll just make another one... I've noticed that most after-market parts bags don't include a lot "extra" parts to replace the ones that do that...
Bottom line is, for me anyway, "if you think you have to use 'em in order to compete, you're probably right"... After all, to paraphrase, "Those that can, do. Those that can't, PURCHASE." (Put down the ax, I'm just kidding... Sort of...)
At any rate, do what ya think ya need to do... Just don't get bent outta shape if I don't agree with your conclusions... That's how I keep my blood-pressure low...