"I'm going back to the Dursleys' once more, because Dumbledore wanted me to," said Harry. "But it'll be a short visit, and then I'll be gone for good."
Dumbledore wanted him to go back to the Dursleys' to keep the protection going between years*, but the protection was going to be gone in a couple weeks anyway, after which they'd have to go to great lengths to extract him safely as or before the protection broke (OK, the original plan wasn't quite so involved or dangerous). It seems like an awful lot of trouble for the extra week or two, which, unlike other years, would not have a lasting effect (keeping the protection going til the next year). Why not just keep him safe at the Weasleys' or another OotP hangout for the extra fortnight and save the trouble?
*: I'm not quite sure why Dumbledore wanted that anyway, it seems self-perpetuating to me. But that's another question.