I had a female tortie (feral) train herself to use the toilet. I have no clue why she chose to do it.....late at night one night I heard the distinct sound of liquid hitting the toilet bowl. I thought it was my husband, but reached over and found him sound asleep next to me. I immediately got up and ran to the bathroom, only to find her running out of the bathroom. This was repeated for a long time, but only in the middle of the night. She eventually quit doing it.
I personally wouldn't want to do this, as they could 1) fall in, 2) leave dirty paw marks on the toilet seat, 3) leave wet foot prints on the toilet seat, 4) leave unflushed items in the toilet and 5) disturb guests that we have over. I've heard of folks training their cat to flush the toilet, but that becomes a game to them and the water bill became tremendous (they flush it all the time).