I don't think he even needs a three point shot, just a consistent jumper from about 15 feet would be great. I think we give up rebounding when our center steps outside the three point line. I guess it would give Favors more room to work on his post game though.
Offensive rebounding is in the area of forcing turnovers with how it relates to winning (in other words, they can be extraordinarily helpful, but they come with significant risks built in and great teams don't usually need or emphasize these aspects of the game). Besides, if you're getting better shots (and if you have a player that can drag the opposing team's most important defensive player out of the paint, you WILL get better shots) you're probably making more which further negates the necessity of offensive rebounding.
Interestingly/unfortunately, offensive rebounding is one of the few clear advantages Favors would have over a small-ball 4 (ESPECIALLY one that might face-guard him if he's able to score on the mismatch in the post).