I think at the time he (Aizen)thought he could pull the remaining espada (Ulquiorra & Yammy) through to help out anytime he wanted so why not leave the big guy to continue resting and gaining the strength need to transform? Plus I am sure with he and his other ex-captains as well as 3 of his strongest and their groupies he really didn't need them.
But I don't much care for the move...this is turning into super saiyan syndrome. Maybe it is I just don't like Yammy and am still pissed they killed off Ulquiorra <_< but it just seems stupid. What is Ichigo suppose to go back into hollow mode again or will he conquer it this time and go to super saiyan 4!?