The basic question is, Given that Muay Thai fighters regularly win against e.g. kung fu fighters when they spar with them, what's the motivation for studying something else? I think there are good many answers: Multiple attackers, attacks with weapons, surprise attacks, something that can be practiced throughout one's life and not just when young, something that stengthens the body without damaging it (one may have to be careful with karate in this regard too, but there's no deadening of the shin for example), the belief that e.g. Sanchin training strengthens and improves the health of one's body, the greater range of options (locks, pressure point techniques, and so on, not just strikes), and just plain interest in the system--as well as the "fit" of the system to the student. Many karate techniques are grappling techniques intended for use against a close-in opponent, not for a toe-to-toe situation.
Muay Thai is an exceptionally effective system and a Muay Thai fighter is extremely well-conditioned. Is that what you want to train for, though, or would you rather train for your likely opponent--two armed muggers who surprise you at night, say? It's not clear to me that Muay Thai is especially suited to that situation.