Nowadays boxing and Muay Thai are the standard striking arts taught in any MMA gym. However, a lot of people underestimate the effectiveness of other striking arts when used in combination with the standard Muay Thai and boxing arts. For example, Anderson Silva has an extensive background in TKD, which I believe has greatly contributed to his kicking technique and his uniquely graceful style. If you want to see Silva using some classic TKD kicks check out his fight with Lee Murray. Another top striker with extensive TKD training is Cung Le. Consequently, I think cross training in TKD could help many fighters improve their striking.