That's not weird, that's a sniper. Seems like a valid profession to want to play.
Also, to note, technically speaking, every misting can burn two metals. Namely, their normal metal (say, Tin) and Lerasium. But short of house rules (no reason you can't) or hemalurgy, there's no useful way to have a two-metal allomancer.
The reason is basically that if you are kind of well connected to Preservation, you get one ability, but if you are a little closer than that, you get them all. Sort of like how email programs have "reply" and "reply all," or how modern English Language has a pronoun for one person or many persons, but not one for just two (Old English does have a dual pronoun, to note).
The real reason is that it worked better for the story that way. Originally, I believe, Sanderson was going to only have mistborn, but that made them seem too common, so he went with something lesser, and still wanted to have specialties, as a holdover from when the first book was a heist novel.