I can't imagine us going into the season not having 3 QBs on the roster. If they cut EJ and started Taylor, it is very likely at some point TT would get dinged up, especially given his style. That could very quickly put us down to just Cassel as a healthy QB with no one we can count on behind him. Simms, or some hastily hired FA QB scares the crap out of me, much much worse than EJ.
Keeping EJ is good insurance, and might yield significant value if he continues to develop.