I don't want to come off as sounding like a know it all or internet commando but thinking about retaining your magazines is setting you up to be stuck in a training mentality.
Hypothetically, in a gunbattle, you wouldn't think about retaining your magazines. I've never done training offered by private trainers but the training that I have had in the military taught me to drop your mag, reload, and eliminate "threat". We picked our mags up in "safe mode", after string of fire.
I can see retaining it if you're behind cover, but otherwise if you have to be shooting and moving because there are so many threats, you can't and wouldn't do that.
I can only see it as a "situation will dictate" kind of thing, otherwise, am I missing something?
"You are fighting for what you can never obtain, and we defending what we mean never to part with." Thomas Paine