I'm not going to lie, i've done it and quite a bit. My last big relationship lasted for 3 1/2 years, I mean we lived together, worked together once and we were engaged, but he was a low down dirty cheater haha, so it ended.
But for a long time, whenever I wanted some I just went to him because it was easy, and I didn't want to.. "add" to my number.
It probably wasn't the best idea but I did it anyway. There's something to be said for sexual chemistry.
Looking back I don't know if I would do it again, but there is something about sex with the ex that is just HOT, it always seems to be better than if you were actually still with the person, don't ask me why, it just does.