I'm gonna say something unpopular: I don't like to rush.

[Vahlnir]

I think focusing on one mob in a pack is playing badly, but I disagree that pulling a single pack is playing badly.

Playing badly? I don't think so either. Playing optimally? Probably not, no. A lot of people prefer optimal play, myself included. Pulling everything you can and AoEing them all down is no doubt the best way. When I'm not tanking (which is actually pretty rare nowadays) and I'm playing as a dps...small pulls like that annoy me, but I also accept that not everyone is able to or is willing to pull wall to wall for whatever reasons they have. As much as it may annoy me, sometimes that is simply how it's going to play out.