May be he doesn't think Spiller is good enough to build an offense around. Or may be he thinks the offense you would build around him isn't the one you need to win a championship. Is it fair to assume the reality of either of those things? I think so.
I also think it's clear (as has already been mentioned here), Marrone wants a guy he can rely on getting 3-6 yards each and every time.
It's my opinion that a back that can consistently produce (there's that word again) pays greater dividends in the wins column than a guy who goes 2, 4, 1, 0, 6, -2, 1, 3, 1, -4, 68, like Spiller does.