My family members all shoot the G21 better than any of the others they have tried, so as far as family members being proficient with the G21, they are more so with the G21 than any of the rest.
As far as my uses for a pistol, the #1 use is defense, I am able to ccw the G21 fairly well. I have tried other smaller handguns for ccw and I never shot any of them that well. So my thinking is, if I can conceal a G21 and shoot it well, then why trust something I don't shoot that well, especially under duress.
For utilitarian uses, for what I'd use a pistol for, hunting, scavenging, woods defense, I can hit with this gun, so it's why its the only platform I have decided to have. If I want to shoot squirrels I bring the Marlin 60 or 870 along for that.
All three of the G21's are exactly the same, same 3.5lb disconnector, same Tru-Dot fixed sights, green front, orange rear. They do have very slight wear marks in different areas of the slides, but if I were to pick one up in complete darkness I would not be able to tell you which one I had. Which is what I am looking for, familiarity. The same thing I want my family to experience should they need one for defense.
If they all lay disassembled on the ground, I could pick up the pieces in the dark, put them all together and know they would feel and function exactly the same.
I really like the .45acp caliber, it is just plain effective. When I had 9mm's I would take a 9mm and .45acp cartridge and just hold them together in my hand and the difference in size is significant. I know, it means heavier bullets=more weight, less capacity, but 13 rounds of .45acp is no handicap.
I have yet to shoot a 9mm that has been as accurate as my G21's in .45acp. A full size S&W M&P CORE 9mm I rented came very close, but the other most accurate handgun I have ever fired was a FNP-45, but I had to concentrate more-so with it than I do with the G21 to be as accurate.
I have owned G17's, G19's, G36, G41, G23's, yes they are all Glocks, but yes they all feel different, even the G41. With the closest matching back strap I could find to match that of the G21, it still felt different. The grip texture was different, the sight picture was different as it had a longer and thinner slide. The decrease in overall weight I could feel and it affected my point shooting.
I have long-guns for other purposes, shotguns, carbines, bolt-actions, so my fields are covered as far as other needs are concerned. I have also looked hard at reducing my caliber foot-print and duplication but that's a whole 'nother thread.