Avatar and signature by me.
The only thing that will make her leg a weakpoint is that her offense would decrease and not that she would die if she got hit there imo.
Since her leg is no longer shielded it's her weak point regardless of how you look at it, if she is missing a leg her offense and defense are both lowered.
It IS about the exposed leg since its the least defended part of her atm, kudos to kuro since that's what he's been trying to point out to you >.> ...
How is it that the leg is the weak point when only her arms and legs are covered with that special steel and the rest of her is uncovered -_- namely the head lol.
A pie in the sky?
She could still fight without a leg but definitely cannot without her head. So that's why I'm thinking she has a secret technique or something (bankai maybe or uber fast shunpo idk) that needs her leg that she would rather use to Aizen and risk being killed. I know it's a OP-ish idea but hey it's possible.