Rondo inability to shoot is due to his Form.

exactly looks like a catapult.. he uses his shoulder to guide the ball or sumthin its frickin wierd
That elbow needs to be pointed at the rim. He also looks like he has too much palm (shooting hand) on the ball, judging from the photo. Don't like where the helper hand is positioned either. Until Rondo improves some of these basic principles, I just don't have too much hope of him being that much of a threat from a perimeter.
I do think the size of his hands is a part of the problem with his form. This makes it hard to get the ball on the fingertip. It also affects how easy it is to position the helping hand correctly.
Not only is the elbow not pointed correctly, but there's too much bend, as well. Again, I think this is partially due to having to palm the ball and not really being able to find that sweet finger tip flip spot.
Despite these issues, I still think he can improve the shot. It'll never look like Ray Allen or Steve Nash's stroke, but he'll have to find a way to be more efficient using his own style.
To me the best comparison to role model as a shooter is Magic Johnson. Magic had giant hands too which contributed to his extraordinary passing ability, but always made his outside shot pretty funky looking. Magic's jumper never got beautiful, but he found a way to become effective enough with it to make teams pay for leaving him alone, and he became a good free throw shooter while always having a bit of a push shot as opposed to a classic outside shot.
J. Kidd is a similar more recent example.