The Vizard are not weak, they are actually very powerful. Remember the majority of them are former captains of the Gotei 13 meaning they posses the ability to use Bankai. They decided to "portray them as 'weak'" because the plot would have been different if Shinji was the one to defeat Aizen, that wouldn't have made it possible for Ichigo to fight him and we wouldn't have seen Urahara and Isshin in action.
I think it was a mess-up by the author and since there weren't a lot of espadas left, but so many vizards, I guess he had no choice but to make them weak. Its proven to be a mistake because on ch.197 of bleach, Hitsugaya states that a captain would "break no sweat" going against the gillian menos grande. Yet in the fake karakura town arc, the vizards destroy the gillians and everyone looks so shocked and amazed. But I guess you could still call them strong because none of them use bankai, even though the majority of them are captains..