Need I bring up the injury/guaranteed money point?
Lets see, Ryan Grant, has played in 17 games, and been featured in 12. In those 12 games as a feature back, he compiled 1281 yards from scrimmage, 28 receptions, and 11 TDs, and averaged 5.2 ypc. On average, that is $291,667 in guaranteed money each year.
By comparison, Chris Brown got $400,000 guaranteed each year to be the Texans' #2 RB.