It's essentially a gimmick. It started with one character (honestly, among the masses of them now, I forget which) with a fang that got really popular. For some reason, otaku loved the character and all the weird things about her, especially the fang. So, anime producers, seeing that otaku are generally blind idiots that follow anything popular, flood animu with fanged side characters designed to be weird, quirky, and sure fan favorites. They're rarely used in the forefront of ads, but they're usually featured in them regardless, in the background.
It's a marketing ploy. Nothing else.