An inconsistent Rondo who seems invincible half the time would be better than a consistently decent but unspectacular point guard against a team that is clearly better than the Celtics. The consistent point guard would be better against bad teams. If Rondo has more good games than bad, then he is preferable against teams that are slightly worse than Boston.
Essentially, we'd probably have a better record with the more consistent point guard.
I think it comes down to what role you see Rondo ideally playing on this team.
If you think he's capable of being "the guy" as he is, then it makes more sense to stay with him and hope that his "Good Rondo" performances come when needed against the best teams, and often enough for your team to win a good number of games.
On the other hand, if you see Rondo as only one piece of an incomplete puzzle that needs to be put together for this team to be a contender again, it might make sense to eventually move him for a more consistent contributor, particularly if by "down-grading" in that way we could get other assets.
Also, part of my purpose for posing this question is that I think this "conundrum" is one of the major reasons that Rondo's trade value doesn't seem to be nearly as high as we all think it should be. When we consider Rondo's value, we tend to think primarily of games like he had today. When NBA GMs (and outside observers) consider Rondo's value, I imagine that they think a lot more about the times that he disappears or plays like little more than a very solid role player.
Indeed, before this season started I got into a Twitter argument with an ESPN writer who actually claimed that Rondo was nothing but a role player who benefits from his teammates. Even citing (in my opinion) Rondo's greatest game as a pro, one of the most amazing NBA performances in recent memory -- the 29-18-13 game in the 2010 playoffs -- was not enough to persuade this guy otherwise.
The idea that Rondo is a role player is a legitimate belief among many outside observers of this team, and I think Rondo's maddening inconsistency plays a large role in the perpetuation of that belief.