Why so much singleback formation?

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earlier this year, no one could stop talking about Mike Sellers. he's proved he's excellent at hitting people, and can catch the clutch short yardage pass, often for scores.

I'm wondering why he's not on the field most of the time instead of just in the redzone. a shifty back like Portis plays best with a fullback in front of him, much like they do in Denver and Seattle. is Robert Royal so much of an asset that we'd rather play him than Sellers most downs?

- how's Sellers at pass protection? is he that much worse than Royal?

- is Royal a big enough downfield recieving threat to forsake Seller's stiffening of the line and safety valve ability?

- what other teams use singleback formation almost exclusively, and are they of the 3 WR or 2 TE variety?