I think you just think higher of Webster than most of us. I think he is a decent, but not great, shooter, who doesn't give you a whole lot more. And in a deal like that, he would have to be the center-piece, as Pryzbilla and Blake are not going to be any more than role player rentals.
In any trade for Allen, we need to get at least 1 player who you can plug into the starting lineup going forward (or at the very least, a legit 6th man of the year candidate). I personally don't think Webster is that guy.
Fair enough and understandable and I do like Webster a lot (assuming he is healthy of course). I saw a high school kid with size and athleticism (he is 6'7" or 6'8") make good progress and improve every single year he was in the league. His career TS% is 53.5% and his eFG% is 50.1%, which for an outside shooter is very good especially when his age is factored in. His three point shooting got better and better, and his defense got better as he got experience (he still has a ways to go defensively).
I just think he is exactly the type of player the Celtics should be pursuing, a good kid, with a solid work ethic, that has improved every year, but that is still a long way from reaching his prime (he is still just 22). A guy like Caron Butler (who you mentioned, but as you also mentioned won't happen) is already 28, so you will get maybe 2 years before he starts tailing off. Sure he is better than Webster right now, but can you say that in three years nonetheless six years from now. If part of the point is to start bridging the gap and building towards the next championship core (guys to put with Rondo and Perkins), you don't go after a Butler type player (he is too old), you go after a guy like Webster (I realize you don't like Webster, but that is the type of player the C's should be pursuing). And it isn't like this trade really hurts the C's chances next year as Pryzbilla and Blake add a significant amount of value to the team, regardless of what Webster does.
BTW, I would love to get Fernandez or Rodriguez from Portland as well, I just don't think they do the trade with the extra guy thrown in.