This topic has come up before. A lot of people say that the Celtics don't need a backup point guard because Rondo can play 40+ minutes per game in the playoffs. I'm not so sure.
In 2007-2008, the Celtics didn't really have a true point guard to backup Rondo. I suspect that was partly by design to force Doc to play Rondo and to prop up Rondo's self-confidence. Those reasons don't hold since then, but that hasn't stopped the team from constructing a roster where Rondo is the only real point guard.
Sometimes, Rondo seems to be pacing himself, trying to conserve energy so that he has something left in the tank for the fourth quarter. We saw in Game 2 vs the Knicks that when Rondo goes full bore, he may get gassed and need to be taken out of the game a bit earlier than usual. We also saw where Rondo was given a game off late in the season and came back playing better.
Rondo needs a caddy so that he can be better rested, within the context of the entire season as well as in individual games. That doesn't necessarily mean I want Carlos Arroyo back next year. I see that Anthony Carter of the Knicks has been a minimum salary guy pretty much his entire career ($16 million career earnings in 12 seasons) and I wonder if the Celtics would be better off finding a similar veteran (maybe one who has the markings of a future coach) and locking down the backup point spot for a few years on a series of minimum contracts.