Didn't San Antonio and Miami and Detroit all try that "Let's bring back the same crew and repeat." philosphy? How'd that work out for them. Bringing back the same crew guarantees nothing and therefore is not the best chance to repeat.
The Celtics have the best starting 5 in the league. As two of those players grow and the 3 others slowly diminish, their needs off the bench will change. Next's year's bench will probably have to provide different things than last year's bench.
It is possible that a bench consisting of players not named Cassell, Brown, Posey, Allen and House will provide more of what this year's team will need. Giddens, Walker, and O'Bryant are young chips that Danny wants to develop. One or two might become something unexpected. In the meantime what's to say that the current team plus Kurt Thomas, Matt Barnes and Starbury(yikes, I hate the thought of him in Green) wouldn't actually be a better team than last year's.