I don't think it's a matter of holding Charles out to save him, I think it's more of a situation where CJ has been too good to bring out.  Charles only goes in to spell CJ for the most part.  When Booker comes back, if CJ and Charles are still healthy, I don't see Booker getting lots of reps.  Both CJ and Charles are too good in pass protection and both are very effective catching passes.  I always am happy to see Charles in there on third down because I know that he will pick up the blitzer and if he can, he will leak out of the backfield and be an option.