Two qbs is okay, if you're down to your 3rd qb he ain't doing much, why else is he no. 3?
But with the varied offenses in the NFL, you really need 3 rbs, if one gets hurt that severely limits your options as the no. 2 guy will be taking all the snaps, too easy for the defense to key on him.
Of course you have the p.s. but if your p.s. running back is good enough to be a starter, even on the short term, he's liable to be poached off your p.s.
If I'm interpreting the p.s. rules correctly, haven't really studied them....