Was the final G.T useless?
it wasnt.it was the only way to weaken aizen so that urahara could seal him.
Considering the fact that Ichigo could break free from Aizen's grip, and shatter that final Cero Hollow Aizen tried to use. I can't help but get the feeling that Ichigo prob could have defeated Aizen without it, although you could argue that the hyougoku prob would have given Aizen another power up. Still, without it, Aizen wouldn't have felt satisfied, and have the hyougoku reject him.
I thought its usefulness was diminished by the scene of Ichigo theorizing that in Aizen's heart he was lonely and wanted to lose his powers. This implied it was actually Aizen's own doing that caused the hougyoku leave him and adds question to whether Ichigo using the final getsuga would have actually made a difference or not.
It beat the villain, so question answered.
Avatar and Signature by Ravona