The NBA is a league of superstars. The only real point in accumulating assets is in either in the hopes of trading those assets for a superstar or in the hopes that one of those assets will become a superstar (picks/youngsters).
That said, I'd be very hesitant to deal Rondo, particularly at a time when teams may question his health. He provides the Celtics some sort of respectability not only for recruiting other players, but for also appeasing the fan base.
Furthermore, while we can debate just how good Rondo is, he has shown at times that he can be an elite player who can absolutely dominate a game. In a league where it's absolutely essential to have a player like this (preferably 2 or 3), I'd be very hesitant to deal a player like that away, particularly if we're not getting a similar caliber player in return.
And quite frankly, I don't see many teams looking to give us a similar player in return.