I think Sproles went underrated in drafts this year considering the departure of Tomlinson and the fact that Ryan Mathews, though highly touted, is a rookie unknown. In a league with return yardage scored at that clip, I would guess he's good for about 60 extra points relative to those RBs who don't return kicks.
I've always felt like Sproles was a very good NFL player if undersized to handle a traditional starting RB workload. Seems to me like everytime he's gotten a chance to "start" he's come up pretty big. He may not be a between the tackles runner, but he is excellent catching the ball out of the backfield and a great outside running threat.
Given his presence in SD I was kind of surprised to hear Norv speak so highly of Mathews's workload. This would seem like an ideal situation to use more of a committee while the rook gets acclimated to the NFL. I guess if you're taking Norv at his word, Sproles's role won't change this year, but I think there's a chance that as the season plays out he earns more of a role depending on his and Ryan Mathews's success on the field.
Either way, in return yardage leagues, he's probably a top 40 back, which is definitely roster material and a bye week/flex fill in.