I don't think the DM should do anything except what he and his group agree on.
I really don't think there's a "correct" answer here except for what works for your group.
I agree. Some groups are probably comfortable immediately importing previously unestablished races into the campaign. That's fine. Others, like me, maybe we built a campaign around certain parameters that don't include wemics and won't suddenly include them just because the Complete Book of Humanoids came out.
I think that's the real issue that alot of people have - feeling like they have to disrupt their established setting to include new "core" material that WotC has published.
Like Dave said, the important thing is to discuss this sort of thing ahead of time, so that when new material comes out, everyone knows what to expect. In my games, we wouldn't be adding stuff just because it's in the new PHB. We might include it next campaign if whoever DM's wants to and can find players willing to play under those circumstances.