Looking at those, not having one in hand, I'm pretty worried about follower tilt when trying to open it up. The only good use I can come up with for those is for non-detatchable mag CA rifles, where sliding the magazine open and dropping in rounds would be easier that breaking the rifle open and loading them into the magazine one by one.
If I want something new and high-speed, I'll buy twice as many PMAGs and keep them all loaded. That way I can just change magazines, which is how many times faster than opening the front and stuffing bullets in.
As for loading, I use the easyloader that came with my Beta to stuff rounds in fast.