It's just that no other back on the roster does what Conner does, but there are people in place that do what McKnight does. Hell, despite his gaudy kick return average, Westhoff threw Cro out there in any spot that they needed a clutch return. Regardless, I think Sparano will sort the back situation out pretty quickly. McKnight's allegedly semi-retarded and he's going to have to learn a new offense on the fly after not learning Schottenheimer's offense in three years. If Powell shows anything, and Conner survives as the blocking FB, McKnight's fate will rest with Westhoff, IMO.
I think the whole position is up in the air beyond the "A" group. Everyone else has to prove they deserve to be on the roster. Powell has the steepest climb, having produced nothing last year, and with McKnight (despite your assertions) somewhat entrenched. Conner has a defined role that needs to find a fit in Sparano's offense.
Powell has a lot of upside still, IMHO, but he's going to have to show that this summer. People pile on him a little bit, but he never had a training camp as a pro. I think he's the guy under the most pressure.
No reason all five backs can't stick. The Tebow thing complicates it a little, too, taking away some of those RB carries. Should be interesting to see how it shakes out.