9MM is the standard handgun caliber for our military forces.
Better to have a weapon in a caliber of the most easily found ammunition...
But only because some piss ant in the Carter administration declared it should be so, in the interests of "Commonality". Many in the military would have preferred a new .45, perhaps with an option for a 9mm in a smaller, more modern, package than the M9, something like the M11, for the GIs who squat to pee, or who need to conceal their firearm (CID, OSI, NCIS) and can use modern ammunition (I think?) . By now a.40 S&W would be better even for those categories.
9 mm would be OK if the military could use expanding ammunition. But they can't. So they tend to shoot through the bad guys, rather than take them down. As long as you can't use expanding ammo, you need a big bullet, even if that means it's slower.
For personal defense, with modern expanding ammunition, the 9 is fine. (Although I personaly only own .45s, I wouldn't mind a 9, in a very small and concealable package, like the KelTec P11 for example).
Range ammo is relatively cheap (and increasingly easy to find) which means I can afford to practice my defensive handgun skills. A lot.
I can't think of anyone who would care to be shot with the combo. Nope. No one comes to mind.