The majority of good promos in the marketplace, at least here, are not postworked renders.  I'm not sure where the idea comes from that if it is a pleasing image, it must be postwork.  If you are counting putting a cutout png render of a figure on a background as postwork, then you are seriously missing the point.

Poser default settings and lighting simply do not do justice to ANY content, no matter the quality, and in many cases can hide a lot of problems, especially when it comes to textures and materials used.  Poser's default lights cast some serious dark shadows that can cover a lot of issues, and hides a lot of detail.  Personally I am much more inclined to buy a product that is well lit and rendered over one that uses default settings for the promo images because I can better see what I am getting and don't have to worry about any hidden flaws.  Shoot, you don't even have to know anything these days about lighting, posing, or render settings because there are a lot of excellent products in the marketplace already to do much of this for you.  IMO bad promos are simply lazy promos.