Felix is too much of a threat to not put on the field w/Marion. YPC will increase for both, regardless of attempts. Felix has a chance to be a unique player in the league, and for many many years. In the Reggie Bush mold, but with the ability to actually handle a full work load if called on. Felix will be a superstar in this league for along time, but here's the difference... Felix is the joker in the deck: a player whos ability you cannot coach duplicate and prepare for. Could he handle the bulk load? I think maybe so. But: I do think his effectiveness would drop drastically if he was given the feature back role. I think he would still be considered highly productive, but his SUDDENNESS would decrease to where it might hamper the offense a tad.
As for MB3. Everyone that is worried about Barber slowing down with carries sees one play in the NFC divisional playoff where he got tripped up at the line of scrimmage on the Cowboys penalty riddled final drive as "proof" that he wears down late in ballgames with a full workload. That's nonsense.
MB3 will be a 1300-1400yd RB with 15-20 TD's running/receiving for at least 4 or 5 more years barring injury. For the next few seasons, late in close games HE will be the feature back in the line up- do not confuse that.
These guys lack of ego is part of the reason they are both terrific ballplayers. They have a chance to BOTH be HOF'ers, and go down in history as a uniquely productive, talented, and dynamic duo of RB'sm and we as fans should not root for one or the other but enjoy both because they are a joy to watch play football.