I've ridden thoroughbreds western. Fitting one with high withers is an issue, just need to find a western saddle with a higher gullet. However not all TBs have high withers. I have a QH right now that has quite high withers, more than some of the tbs I have had.
I used to ride my TB western. Never had any problems. Like horseluver said, you just have to be careful to use a saddle that can accommodate higher withers (and narrower backs, usually. Depends on how your horse is built). Proper fitting tack is kind of a no brainer. :P
I loved my first tb ex racer who was then turned into a western pleasure horse... she loved the western saddle and hitting the trails. Just find a saddle that fits and your ok... if you need help with the high withers find someone who fits saddles to get you the best fit :)
My 18 year old OTTB was green broke to english when I got him. He is now neck reining and a really nice trail/ putter around the farm horse. I already had a saddle that fit big horses, so that wasn't a problem for me. He has a nice big, ground covering stroll, and he now keeps a low head set, he looks very nice being ridden western. Posted via Mobile Device
My first horse an ex racer was a great trail horse and could bury his tail and do an incredible roll back. He was like riding a spring, he could coil his body in beautiful floaty collection or stretch out and relax.