I've always been a big proponent of making teams matchup to you, not matching up to other teams.
Yes, in matchups like that, the C's give up size. However, they also get speed and spacing matchups at almost every position. Bradley is faster than any 2 in the league, Ray is faster than almost every 3, Pierce is faster than any 4 (and a nightmare to cover).
And really, only elite players who are going to get their points anyway are going to exploit those matchups. For instance, is DeJuan Blair or Matt Bonner going to abuse Paul Pierce in the post if he's playing the 4? I think not. And I think it's actually a gain for Boston if the Spurs were ever to away from their offense to try that.