Alot of them do shoot a Glock. But several don't. Hell, I watched Ernie Langdon run a Beretta M92 like it was on fire last year. The truth is, for most of those guys, it's just a tool with no emotional attachment. The ones that do run a Glock will tell you it's because they just plain work. No dispute there. But to say that the M&P is a lesser gun just because Smith makes other guns, isn't true. It's been around for 1/3rd of the time that Glock has and has evolved, just as Glock has. Today, it's a solid gun. Extremely reliable, better ergonomics, ambidextrous controls, arguably more shootable and similar accuracy. Don't get me wrong, I'm not knocking Glock at all. I just cringe when people get romantic about a particular brand.
For the record, the Sigma was a Glock copy. But the M&P is wholly different and in some ways, improved.
I'd rather see folks make their arguments with quantifiable advantages or material differences versus the "other guy". I know a couple of industry guys that shoot M&Ps too, but not nearly as many. And a couple of them have upwards of 25 or 50k rounds on a single pistol. So the M&P has the chops to compete with Glock. And the original Pistol training.com torture test proved that. But Glock has a long track record and it has earned it's reputation for dependability too.