I have posted this before, and I am NOT a party pooper...but, there have been MANY instances of cats/kittens drowing in toilets. They go to drink, fall, and because of the way the toilet is shaped, with VERY slippery porcelin walls, they can't get back out.

I just don't think it is worth it, safety wise, to let cats/kittens drink from the bowls. AND, depending on the germs or disinfectants one uses to clean the bowl...well, it just doesn't seem healthy.