I think the only benefit would be saving a little CO2 from the bottle. Naturally carbonating in the keg is going to leave some extra yeast at the bottom too meaning a few pulls before you get to the good stuff.
As far as "conditioning" goes. I think that if you prime the keg and let the yeast do their thing it sits on the yeast longer and conditions. If you don't filter your beer and force carb it, and leave it in the keg for the same amount of time as the naturally carbed keg I think they would be equally conditioned. In fact the force carbed keg may be more conditioned because there isn't nearly as much reproduction and the yeast do not have to clean up after the re-fermentation. Might be worth a side by side comparison.