The cover is primary designed for impact protection and to keep debris out of the magazine during periods of long term storage.  

It does also push down on the rounds to unload the feedlips but this not required to use if storing the magazines loaded.

In lab and realworld testing PMags have been kept loaded for over a year with no cover on and no deformation of the feedlips has occurred.

If you do have the cover on and need to use the magazine the edge of the magwell will pop it off in under a second even under stress.