One of the reasons I've seen to not use the slide release lever but to slingshot the slide manually was from Clint Smith.
I'm paraphrasing here (been a long time since I read/heard it) but he said something to the effect "Not every pistol will have a release lever in the same position, working the slide manually with the other hand reloads every pistol available, regardless of mfg."
I don't know - I'm not a tactically low speed guy but it does make sense at first blush. I do question tho that if you're using a battlefield pick up pistol so to speak, what are the odds you've also picked up spare mags. If you had time to get spare mags, I'd think you had time to familiarize yourself with the pistol.
Dunno - just thinking out loud.
