*Disclaimer - I only do the trade if Webster's foot injury is healed and he is back to normal. If he is still injured I would also do the trade swapping Outlaw for Webster, though I'm not sure Portland would do that since Outlaw is expiring and is a better fit for what is left on their team.
Why for Portland:
Portland needs to move some of their non-star but quality players for more elite level players. Allen is definately older, but that team needs a good quality veteran that can still play. Allen, Bayless, and Roy would be a solid backcourt combo for them. They would still have Aldridge and Oden down low (and could re-sign Fyre and also have the mid-1st round draft pick) and would have Batum and Outlaw (or Webster) as the wings. Fernandez and Rodriguez would also get some backcourt minutes.
Why for Boston:
Webster could start next to Pierce (the C's would be big, but that would work out fine against almost every team). He is still very young and was showing solid progression every year until the injury this year (plus his contract is very reasonable). Pryzbilla is the perfect backup center. He rebounds well and is a solid defender. He also doesn't mind coming off the bench and could start if Perkins is ailing. Blake, like Pryzbilla, is the perfect backup for Rondo. He is an excellent outside shooter (over 40% from three the last two years) and you wouldn't feel badly if he was forced to start and/or play a lot of minutes (if Rondo goes down) since he has that experience and has done well in that role.
The C's would also have more flexibility going into free agency this year and will have a lot more flexibility going forward. It significantly strengthens the C's bench, though they do lose some in the starting lineup, but with Allen's age I expect a continued regression from him anyway. Blake is expiring and Pryzbilla has a player option after the year (I expect him to take it), so Webster is the only guy signed long term and he is making about 4.7 mill a year (so is movable down the road anyway).