Well MMA isn't actually an Art, its a competition using multiple Martial Arts together.
Muay Thai is perfectly fine to be used as a striking Art. It trains the practitioner to use many of the aspects of striking that are possible to be used in a sport environment.
In my opinion, learning Martial Arts such as Muay Thai, Judo, Brazilian Jiu-jitzu, Karate, etc. separately is a much better way to learn the fundamental techniques involved, because its the entire focus of that day of training. All you do every single day of Jiu-jitzu class is learn to roll, and develop better technique, a deeper gas tank, and overall skill. An MMA gym, also in my opinion, is a great way to put it all together in a controlled environment. For example, you know how to grab a clinch and slam knees home from Muay Thia, you know how to sweep from Judo, and know how to roll and control the opponent from BJJ, and in the MMA gym, you can learn how to use all of these aspects in conjuction.
But this is all my 2 cents, hope it helps!
Sig courtesy of that photo-matic magician limba
I went out the way that I fight. I went out on my shield. Thatís it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit