There are enough counters to being dominated that when it does happen it should be a memorable experience.
I usually have the Bad Guy tell his (or, usually, her) dominated minion to slay a specific party member (the wizard, for example). If it's a player I know is good and I can trust to play the part, I'll let them do whatever they would do to kill their fellow PC - there's no reason they wouldn't do what they always do facing a matching threat: fight to the best of their ability. they should use every feat, trick, and combo in the book to slay their erstwhile ally.
In the rare case in which I had a player reluctant to do so, I'd probably just "NPC" the character; I've never had to do this, though.
My view of dominate person is that it's among the most potent, dangerous spells - when a PC is dominated, he should become the sole focus of the rest of the party. Why nerf it on the (very rare) occasions when it happens?