I agree with what a lot of people have already said: we do need some big man help, however it is probably better to come through free agency. Quite simply, it's easier to get potential superstar talent at the 2/3 spot than at the 5 spot, and I think that's what Ainge did with this draft. That's not to say that they will be superstars, but maybe they'll have a chance down the road. With big men, there's an even smaller chance, which is why he passed on Jordan.
Plus, with Walker and Giddens, they could potentially step in next year and play Tony Allen's role off the bench. Why not just let Tony Allen do it? Well, I don't think Ainge wanted to commit a sizable contract to him given his injury history and the fact that we don't really need him right now. Maybe Tony Allen can be a starter someday, but we don't need him to do that today. So there's no real point in paying him 3-5 million a year to sit on the bench.
Ultimately, as others have said, we should be able to lock up someone who wants a championship. In the regular season we should be fine as we are now (assuming everyone's healthy), but I think we need at least one more legit 7-footer to cover all our bases for the playoffs.