He was re-signed in the cheap, at his actual and current worth in the league, vet minimum. When they cut him they didn't seem to worried with EJ backing up TT.
With what transpired in the last two weeks, it was a shrewd move by Rogue Whaley.
Maybe you're overestimating Cassel's value as a player/QB. He was signed to be the disaster option in case TT and EJ failed. They didn't, they showed they were equally better or just plain better than him up to this point.