Well, for what it's worth, I'll give you my opinion.
I honestly think, for pure stand-up, Muay Thai is the best martial art, with boxing as a very close second. I'm biased towards MT because I had the privilege of having my ass kicked by a Thai boxer from Thailand.
In Muay Thai competition, you will usually only see the teep (front) kick, and the roundhouse. And this isn't because Muay Thai isn't "advanced" enough to use kicks like side kicks, spin kicks, etc. (When I was more naive I used to think San Shou was better because it had more kicks)
The reason behind this is because these two kicks minimize your chance of exposing your back and hamstrings. In MT competition, if I'm not mistaken, there are very few spots on the body you're not allowed to hit, and the back is not one of them. You can be kicked in the kidneys I believe. But if I'm wrong, you can still be kicked in the hamstrings, which will take you out pretty fast. You can still see other kicks used in MT competition, but they are more rare for this reason. If you just use the teep and roundhouse, you can stay facing your opponent.
What's also really convenient is in Muay Thai, your front leg needs to be squared off with your opponent, so you can properly check leg kicks. This carries over really well into MMA because any good wrestler will tell you that you need to be squared off to properly defend takedowns.
The kicks from TKD are definitely awesome. And I don't think there'd be anything wrong with putting some of those kicks into MMA. But I would never study pure TKD for stand-up. You'd need to mix it with something.