It's a gimmick. Look at Leupold and other quality scopes - do they have something similar to posi-lock? No, of course not.
I have used Leupold Vari-X IIIs, VX-IIIs, Elite 4200s and 3200s, Nikon Monarchs and Pre-Monarchs, and even Burris Black Diamonds without posi-lock. I have never had any problems with them.
Keep in mind that my Elite 3200 sits on top of a .375H&H Mag! This scope has help up perfectly!
Forget about Posi-lock - it's a joke.
Zachary