Going back to the couple of people who have espoused the whole "the Celtics have too many numbers retired!" argument . . .
I think a fair argument is that, though opinions may vary, anyone who's actually enshrined in the Hall of Fame should be immune from debate as to the validity of their number being retired. Sure, some of them had careers beyond the Celtics that buoyed their HoF support (DJ, for example), but I don't think the ones whose numbers are retired in Boston are really ones who came here for a year or two at the end of their careers. So, if we eliminate the HoFers (and Satch/Red/Walter Brown, who are in the HoF but not as players), we're left with:
-JoJo White (#10)
-Don Nelson (#19)
-Cedric Maxwell (#31)
-Reggie Lewis (#35)
-Jim Loscutoff (Technically the same number as Cowens, but, hey, he's up there as "LOSCY")
Of those five, I think that we can reasonably defend JoJo without much effort, and eliminate Loscy because Cowens' jersey retirement made it moot. So that leaves us with three:
-Nelson (who spent 11 seasons with us, and arguably began the "sixth man" trend because he was playing behind a lot of talented players)
-Lewis (who went to Northeastern, was beloved by the fans, and who died young; it's a sentimental pick, of course)
-Maxwell (the arguments for and against have been listed ad nauseum)
So, really, in my eyes the only "illegitimate" retiring was Maxwell's, and even that is at least debatable. I don't think it's fair to NOT retire a number just because a team has had a lot of talented players. I mean, no one would say Pierce shouldn't make it in while comparing him to Bird or Hondo or anyone else.