He's been part of a two-back system sharing touches with Fred Jackson when both have been healthy, a rarity anymore in the NFL:
Who starts is only an indication of who's on the field for the first snap. Sometimes they've both been on the field simultaneously, sometimes not in single back or with a fullback. Spiller's been more than a 3rd. down back but with fewer touches than a feature back.
Spiller looked bad when he came back at the end of the season, but that might have been because he came back early for the playoff challenge. If he practiced before coming back, it was only for a day or two.
Spiller has been injury prone but a dynamic player when healthy. If healthy and cheap he'd be a good addition. If healthy though, he won't be all that cheap. He's still in negotiation with the Bills but there seems to be some distance between the bid and the ask.