I think there's significant value for all serious defensive students to be familiar with as many types of commonly available firearms as possible. You should know how to unsafe a 1911 or a Beretta, or an M&P, or an HK. You should know how to load, and more importantly CLEAR anything that might fall into your path.
Having said that, I continue to agree that, in order to attain peak efficiency, the fewer firearms I train with, the better I perform with the ones I use. It doesn't help ME, in my journey, to switch between a Glock and an LEM, and a DAK, and a DA/SA, and a 1911 or a P7, or whatever. The more things I have to deal with, WRT firearms function and handling, the less attention I can pay to cleanly pressing the trigger without disturbing the sights.
I recognize that there are folks who have the time and resources (money, range availability, ammo) to maintain a very high level of proficiency with a bunch of different weapon types. But, that's not me, and it's my experience that MOST folks are actually a lot more "casual" about their training than the Interwebz might lead you to believe, and I think those folks are doing themselves a disservice. Again, YMMV...