How many people are going to give it a good try anyway? Worse still, if it doesn't fit, they're either going to try and fit everything into two lockers or pay for a large one.You can fit a lot in one small locker, but some backpacks are just not going to fit in a small at all.
This simple locker interaction has at best resulted in taking twice as long as before.
It's also a problem that can't be fixed by using PSAs.
Edit: What I'm amazed hasn't been discussed yet is buying a bag that can be securely attached to a railing. This is going to be a security nightmare if it catches on.