You can often find great RB's that drop or slip through the cracks. For that reason I always like the idea of keeping the backup RB's young. This way you keep adding and eventually find a guy. You stick with a big name for backup and he is clogging the place up. Have 2 young guys back there behind Charles and you add some youth to your team and save some on the cap. Plus you allow for the chance that the guy turns into something, instead of hoping he has one good year left.
6 of 12 QB's in the playoffs are 1st or 2nd year players. Oh how scary it is to go young at the QB position.