I'm just throwing this out there because I see this response all the time. And actually, I'm not necessarily focusing on MBIII/JJ... the same observation is true for other RBBC situations like Foster/DW and FT/MJD. However, for argument's sake, forget what you know about LT and how he's performed as a featured back: I'm curious how you think a tandom like Tomlinson/Turner would perform splitting carries? I'd say SD would absolutely be a top-10 rushing team, perhaps slightly less productive than using LT as a featured back. That said, would the mere fact that such a tandem proved to be successful, in of itself, dictate that San Diego is definitively better off sharing carries than feeding LT the ball as a featured RB? I'd argue that many NFL teams could be successful splitting carries among RBs, but in the end most choose to give the ball to the more productive player. For some reason, Dallas chooses not to, and no one really knows how that may be impacting them negatively. IMHO there's no basis to believe they wouldn't be equally successful, or moreso, giving the more productive player more carries. They could in fact be setting themselves back by giving JJ 10-15 carries at 3.9 YPC, despite their relative success considering that sandbag.
So yeah, maintaining status quo could be a productive move. Maybe its not the best move, however.