A well known tactic of advertisers is to appeal to their largest demographic, and the largest demographic for MMOs just so happen to be anti-social teens going through puberty. And what better way to appeal to those kiddos than with large breasted, scantily clad women in "armor". This can range from tightly dressed to just hardly covering the genitals and tits.
Below happens to be one of the more slutty MMO ads I've seen in a while, seriously her tits are about as big as watermelons and for god knows why she's in a bikini despite the fact she's clearly shooting a bow and arrow. What's the sluttiest MMO ad character you've ever seen? And I'm talking SFW MMOs not one's that are particularly explicit.