He has only used his Bankai once! and that was when (i forget which captain explained) they back tracked Aizen's plan saying that once he learned his bankai he wanted to show everyone and invited all the captains and lieutenants to the court garden to show it off and they all explained it was mystical illusions and water effects. That was the only time he needed to use bankai. its true we have never seen it in action in the anime or manga.
Aizen has hogyoku plus he was going to create his own zero squad leaving the real zero squad in the dust cuz by only would they have to fight themselves but aizen as well the Quincy would have been defeated by him aswell fullbringers are just weak