I have a feeling that what I'm about to say is beating on a dead horse since I only read a quarter of this huge thread.
There are situations in fights that call for Muay Thai, and there are also situations that call for a speedy TKD snap kick. Five years after quitting TKD, I can say that I'm glad I learned the art. It built a great foundation that I could build from and gave me dynamic leg flexibility that bleeds into other arts. There are a few useful weapons I took away from that art that only add to my arsenal.
When sparring, whether it's limited or full contact, I find appropriate situations for a few TKD skills: a quick high front snap kick is a very simple but useful tool and is quite damaging. The jumping double front snap kick can throw people off balance, and has some knockout potential (doesn't Anderson Silva actually use this from time to time?).
Everybody knows about the back-spinning kick; this move can be likened to planting a bomb. If you don't know what you're doing, you're going to get yourself killed (knocked out). Then there's the hook kick; you need great footwork and tremendous speed to use this and when applied appropriately while side stepping, the hook kick can actually do some damage as it may also confuse your opponent. In my opinion, speed is the most utmost important thing for a TKD student... if you're going into a fight with just TKD kicks and average kicking speed, you're going to get knocked out real fast.
When it comes down to it, I ultimately feel that Muay Thai is the superior art in terms of overall effectiveness. Yes, yes, it's all up to the practitioner, but from my experience, Muay Thai gives you a ton more efficient weapons to add to your arsenal when it comes time for a real fight.
Lastly, I know what everyone is talking about in this thread; the arrogant TKD X-degree black belts who are completely ignorant to the effectiveness of other particular styles. Unfortunately, they've helped give TKD a bad name.
I did tae kwon do for almost five years, obtaining a first degree black belt and a box full of gold medals from competitions. I was lucky enough to train under a great master and my Dad trained under the same guy 20 years earlier. He was very open-minded when I continually questioned him about other martial arts and how to combat them, and he wasn't afraid to tell me which kicks are bullshit in combat, and the few that actually have practical uses. In short, he was able to distinguish between competition and combat and that is what made him stick out from the McDojos that litter my city. In a perfect world, everyone could find a master like this in whichever art they pursue.
I'm very glad to see someone say they are glad they took TKD. My teacher draws these distinctions also.