I think letting him walk is the right move. I honestly think Sproles is the product of the system out there.
That passing game and those weapons are so deadly, you have to drop everyone back in coverage, and little ol Sproles and his speed makes for a great checkdown for Rivers, but that's really his main game right there.
Sure he can do some running too and kickoff returns, but once SD gets a real RB in there (and a real FB) that offense will be damn near impossible to stop.
Ive said it before during the season, and I'll say it again. The Chargers in my eyes, not the Saints, have the best weapons and system on offense in the league.
Once they get a run game they'll be scary good offensively.