CBS Sportsline has a ranking of the Shooting Guards and Wally is ranked 15th. Not too shabby considering the number of Shooting Guards that are in the league. Kobe and DWade ranked 1 and 2. They had this to say about Wally:
15. Wally Szczerbiak, Boston: He seemed to fit in well after coming over from Minnesota, not that it translated into team success. He and Paul Pierce could form a productive wing combo, defensive proficiency notwithstanding. But Szczerbiak has to continue the positive momentum enjoyed last season following a few down years.
I still haven't decided whether Wally should start or come off the bench. If they are playing well, West or Allen could start at the 2 and bring Wally off the bench. It would do 2 things. First, it would be better for Wally's knees and help prolong his career. Second, it would give us a veteran leader on both the first and second teams. I expect Doc to start Wally, though as that has been his tendency. [More] source:CBS Sportsline
I think both Allen and Delonte can add good things in the starting lineup. Delonte can be that kick out option for Telfair to pass to on penetrations (when defenses colapse on him), and Tony is an athletic slasher who can work with well a creative playmaker like Telfair.
And like you mention, having Wally on the bench gives us a veteran scoring presence to hold down the fort with the second unit.
I think whether Wally comes off the bench or starts would depend a lot on how him and PP adjust to the new pgs and style of play. If we always need a proven scorer or passer on the court then maybe he comes off. If we need someone to take pressure off PP in terms of double teams maybe he starts. if the pgs don''t pass much he starts and plays as many minutes as possible. If we need more defense Tony Allen plays more.