If you are not a lefty...I know of one really good reason to do it.
I have a co-worker that carries a 3" 1911 for his CCW. It has an ambi-safety (...or did!) and more than once, I caught a glance of his pistol in a OWB pancake holster, with the safety off!
Apparently, getting in and out of his truck or bumping up against something, caused the safety to get moved to the fire position. If you don't have it covered up...this does not seem very hard to do. Rub up against something...even with a shirt or light vest over it and you can easily snick off the safety because it is out there to be moved.
Nice....then..unless you check often...you are carrying around a 45 with no safety on. Not a comfortable feeling, if you ask me.
With the safety up against your person, there is MUCH less chance of this happening. I know from personal experience that I never accidentally moved the safety on my own 1911 while carrying it concealed.
So yeah, if you are not a lefty...I'd avoid the ambi safety.
- brickboy240