I don't know if anyone else noticed it, but David wasn't in the group for very long. I mean, they find out about David getting the box and the summit meeting around the same time--give or take a day-- and after talking to Erek, they head over to David's place to steal the box. The next day, David talks to Marco and tells him about the email, and Marco ditches school to steal the box. David becomes an Animorph later the next day, and they pretty much leap into the attack on the resort the summit is taking place. They escape the very same day and the next day, David is outed as a traitor and seemingly kills Tobias. The rest of the trilogy takes place over the course of a few days and nights. So David is really only part of the group for about a week, give or take a few days.
I don't know about you, but it seems like the whole ordeal should have taken a little longer.