There are good trainers and bad trainers. I've been to Gunsight, LFI, and DTI (Farnam) classes; all 3 taught the slingshot method. I used the slingshot method until I started competing and that's when I started using the slide stop.
The "theory" of why they don't teach the slide stop thing is two fold...
1 - In a gun fight you'll be under the tachy-psych effect (fight or flight) and you lose fine motor coordination, and thus, you may not be able to hit the slide stop.
2 - If you run the gun dry, slam in a new mag and hit the slide stop in one very quick motion, you could release the slide before the mag is fully seated and end up not chambering a round. Now at IPSC matches, I cant count how many times I've seen exactly that happen; I did it myself a couple of times when I first started using the slide stop. I fixed the problem by only dropping the slide stop with the thumb of my left hand, rather than using my right hand thumb. Since my left hand has to seat the magazine, there's no threat of getting the timing off.
Since I stopped competing, I have gone back to the slingshot method.
