Signing a contract and making the team are two different things!
I think Bell is a GREAT gunner, but as they improve the depth of their secondary it will be harder and harder to keep him on the roster. At some point they may have to give his roster spot to a developmental defensive back, like Ramon Walker, or DeMarcus Faggins, or Glen Earl, or Jamaal Lord, who hopefully will also be pretty good on special teams.
Also, they have Derrick Vaughn to consider. He played well as a gunner. Is he a better back-up in the secondary?
As the Texans imrpove their depth, it will be more and more interesting to see how the Texans manage the bottom of their roster.