The Texans need two starting-quality RB's for several reasons.
1. Running backs get injured frequently in the NFL.
2. Davis hasn't proved he can play a full season at any level without getting injured.
3. You want to split carries to some extent to keep the starter fresh. If there is a significant drop off when the second stringer is in the game, the defense won't need to defend against the run and can tee off on the QB.
4. A team coached by Dom Capers is going to emphasize the run and have more running plays than the NFL average.