In MMA there's been a few, not in Muay Thai though, and lets be honest, if they're mixing striking arts, they'll most likely compete in MMA instead of going straight to Muay Thai. But if they did go to Muay Thai, we don't know what would happen, but I would speculate they'd be competitive.
Pettis, Anderson Silva, Uriah Hall, Mirko Crocop, Edson Barboza, Benson Henderson, Cung Lee, Bas Rutten, (and Joe Rogan haha) I'm quite sure there are other fighters I haven't mentioned. We've seen Tae Kwon Do/Karate style kicks effective in fights, we know these move works, and though I don't think they'll make it to Muay Thai though, because of how traditional Muay Thai, there is also the fact that fighters who mix styles seem to go to MMA or Kickboxing. But if you got a Guy like say... Saenchai, and gave him some TKD/Karate kicks in his regime along with all his Muay Thai skills set, I feel that could only help him be more unpredictable.
I agree... to some extent. People say the same about boxing, but yet we know a shitty low level MMA fighter (even one with mediocre grappling) has a really good chance of taking down even a high level boxer and then wrecking him on the ground. A top boxer can also be wrecked by a mid-level Muay Thai guy even standing up. Once you learn certain things to the point you keep doing the same thing over and over again in your career, it's time to expand your skill set. And though kicking is of no importance to a guy looking to compete in boxing, and submission grappling is of no importance for a guy seeking only to compete in Muay Thai.... A higher repertoire of kicks, especially ones that Thai don't train for or see often, can only help. I can't find the videos by themselves, but in those shitty Knock out compilations I've seen at least two decent Thai fighters get knocked out by kick-boxers (who most likely come from TKD/Karate Backgrounds) via spinning heel kicks.