Frankly, I think his jump shot is too developmental delayed for Rondo to ever be more than average at it. That is, it all one had to do was practice, then almost anyone could become a great shooter. But that simply is not the case. For instance, I could dribble a ball everywhere for the next ten years yet, I don't think I have the hand eye coordination to get to the level of Rondo (for instance). In other words, like every other skill, shooting is also based on inherent ability. And for some reason, Rondo either did not develop it in time or he just never had it.
Shooting, in my opinion, will always be Rondo's achillis heel and the thing that will prevent him from ever being great. But that's okay since I think he will be very good for a long time.