A sensual cover can sell just by itself. In video games it's an absolute truth, with adolescents and young adults always interested in the new Tomb Raiders and such, drooling at the thousands of polygons used to render Miss Lara's chest.
In MMO games it's not so different. Marketing executives usually resort to beautiful girls to adorn ads and cover shots, trying to appeal to the significant 18-34 year old male demographic. And don't even get us started in free-to-play Asian MMORPGs, whose usual biggest selling points are the attractive artworks. MMORPG Gate observes some games that used these strategies.