A couple of points. 1. The slide stop is ambi on the M&P so that left handers can engage it easily. 2. Glock from day one has trained that their slide stop/catch is to be used as that only, not as a release. You will never hear them refer to it otherwise.
I don't know if I speak for all southpaws or not, but after about 35 years of engaging the slide stop with the tip of my left index finger,and never using it to move the slide forward, I'm pretty sure that I would ignore that device if it was suddenly on the right side of the pistol. As for Glock, you are 100% correct. When the FBI went to Glock, they (the FBI) insisted that the slide stop lever be easier to manipulate, hence the availability of the "extended slide stop lever." My understanding is that the addition was much protested and debated at Glock, Inc., and was almost a dealbreaker on Glock's part. I kinda like 'em myself, and have them on all five of my Glock pistols.
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