To clarify, obviously it is not against the rules to attack your ally when the power says the target is one creature. But to make that attack an auto hit *if you want* is bad news.
For example, until recently I had a Gnome Illusionist with Phantom Bolt as an At-Will. We also have a Kalashtar Paladin in the party. If I wanted, I could attack him with Phantom Bolt, do 1d6+5 damage, and slide him 1. Were I guaranteed to hit, I would know that scooting him out of the aura of 3 Chillborn zombies would save him 15 hitpoints, at a cost of 1d6 damage (since he resists psychic damage). If it is guaranteed to hit, I would do it every time, and that would be. . .stupid. If I only have a normal chance to hit, I would probably choose another action that I know will be successful. Always being able to slide him out of danger from 10 squares away doesn't work for me.
Jay