When outfitting 30-40,000 cops, it could turn into a logistical nightmare if everyone had something different, I agree with you there. But it doesn't have to be that hard; instead of a dept. buying duty gear, they could accomplish something similar with a voucher program or uniform allowance, and the same thing for range ammo come qualification day. The department would essentially be spending the same money, but the cops would do the shopping. I think the good thing about a policy like this is it involves the officer in the process a lot more.
Down here in Florida there's a ton of agencies (pretty big ones too) who require the individual to purchase their own gun. Usually it's from an approved list of firearms (some say "Choose from these 8 makes and models," others say "anything chambered in X caliber"), but some don't restrict it at all. Some also issue a standard pistol, but let you choose your own if you want to carry that instead. I've heard of a few guys that prefer the latter method because when their gun is taken in a shooting, they still have carry options.
Glock, 1911, Springfield, H&K, S&W and others all make great guns, but everyone doesn't always like them the same.
Almost every single police shooting is more of a one on one type deal, and I've never once heard of one where people were throwing mags to their friends who ran low. Even at North Hollywood, the Ceres shootout, and similar incidents where a lot of cops were on scene firing a lot of rounds, this scenario doesn't come up.