FWIW I have a black belt in TKD and learned a great deal about balance, technique, and discipline. But I would get my a$$ handed to me in any sort of combat. It depends on what you want and what you put into it. I remember guys in my classes that could snap their kicks so hard it was intimidating, but I also remember guys who were way above me in rank that could not punch their way out of a wet paper bag. I did love learning the forms and the flexibility that came with constant practice. If you want to fight look else where, if you want to see what practicing a Korean martial art is like than try it out. As with all disciplines you get out of it, what you put into it......and a great teacher can go a long way.
I would not say that TKD is useless. Alone it's not the best when going against a person trained with MMA. That's the beauty of MMA, it's mixed martial arts. Combining various martial arts together seemingly will always lead to better, more applicable in the real world and combat sports, fighting knowledge and techniques. If a person can bring TKD knowledge into their MMA skill sets, then I think that TKD can be applied to attain effective results. I mean, look at GSP and Lyoto Machida. Both of them apply karate to their MMA skill sets, and they have been effective. Karate was once thought useless in MMA.
A thousand men lined up can be killed in a single day by
a man running with a sharp blade. -- Klingon Proverb