I have grappling shin guards but i do straight MT with them as well. They will work fine if you get a good pair.
I have these:
MMA Amateur Grappling Shin-Instep Guards from Combatsports.com
and they work perfectly. Of course if i was going straight MT and against some heavy hitters id probably get a pair of straight sparring windy or twins insteps.
If your not going to be cross checking advanced muay thai guys and if your going to be bringing it to the ground the grappling shin guards work fine.