It's a financial argument in the sense that I'd rather have more guns, than more calibers of guns. I'd also rather shoot my guns more than an obscure caliber. I own no gun that I don't shoot a lot, and I don't want to buy a gun that I won't shoot a lot.
To me, there are few tasks short of concealment (.380 or .25 could serve this purpose) or hunting (.357, 10mm, .460) that 9mm won't serve me. Granted I am interested in hearing arguments for owning calibers such as .40 and .45 that are common enough that I'd own a single firearm chambered in it. I know it might be boring to somebody else, but my guns aren't and 9mm isn't boring to me. So, I want to hear mostly practical arguments for upgunning.
Self-defense, concealment, and range guns are guns I already have in 9mm. I'll probably buy a few more range 9mm's, no matter what I do, and maybe a lighter 9mm... or a .380.
I'd still shoot a .380, .357, .40, .45 ACP, 10mm, or .460, just don't want to own more than one or two examples in something non-9mm, and I'd want them to do something 9mm can't. A revolver that could shoot .40 and 10mm would be awesome for this purpose because I'd get two calibers out of a single firearm. Too bad there aren't any current production 10mm revolvers out there, so I'd have to find a used one or have one made full custom. Most of my guns are custom builds, and maybe this will sound weird, but I still look for guns that will take less work to get them to where I want them to be. A .460 is pretty sweet, because the S&W triggers are great out of box, and all I would do custom-wise is send the cylinder in to TK Customs to cut it for .45 ACP and possibly replace the grips. That and most .460's are ported, and I just know I'd end up porting a non-9mm gun. (Most my 9mm's are ported. I am a nutter like that; don't ask.)
Heh. You guys are right. It is cruel to me to not put "All of the above" as an option, but I find it more interesting to hear your answers if you had to limit the list. It makes you guys think, and then I get some data a back from your responses. And frankly, that's what I'd like to do: limit the list. If I only own one or two non-9mm pistols, ever, I'll be happy.