That's been pretty standard practice at every table I've ever played at. I know that I don't want to deal with any more creatures in combat than I already have to; DMing is complicated enough without having to run a swarm of summons and a wizard clone. To say nothing of the irritation and time-wasting-ness of rolling against myself. It's even worse outside of a fight, when I have to talk to myself. If I want to do that I'll send my players home and start writing prose.