I think Rondo has to take that 15-18 footer. If he doesn't then the defense won't play him for it. If Rondo's defender doesn't play him for it, then he is sagging off him making it hard for Rondo to beat him to the hoop for a layup. So if Rondo isn't shooting and he can't drive, then he's ineffective.
So he needs to take that 15-18 footer for two reasons. One, to score points because he can hit it at a pretty solid percentage (at least seemingly the same % that Baby attempts about 5 of which per game). Two, to make defenders start to respect it and play him closer which will open up his drive.
I'll keep saying it. Rondo, you have to take that shot. Not every time. If there's the potential for something better, then hold off. But you need to take it once in a while.
Unfortunately, players won't guard you tight simply because you "take" a shot... you have to actually MAKE shots before there's a change in the defense.
To make an obvious example (I feel like I'm beating a dead horse here, but this post has already forced me to repeat what I just said) Shaq could pull up from 3 every night for an entire season and nobody would ever actually go out there to stop him. The defense WANTS him to take that shot. And it's the same way with Rondo!
His jumpshot EFG%, according to 82games.com, is .382. That's AWFULLY inefficient, and it blatantly hurts the team. And it doesn't force the defense to do anything about it (in fact, it almost bails them out). And, like the poster above stated, Rondo is so good that 90% of the time he can find something better to do with the ball.
There's just no reason for him to take that shot, in the playoffs, at this point in his career, with anything more than 5 seconds left on the shotclock.