The problem with using a 3D paint application is that it doesn't yield the same effect as texture baking. For instance, let's say you want to put some fake AO into a texture. With texture baking, it's a snap: Just render a procedural AO shader and bake the results into a texture map - Done.

By hand, depending on how you've laid out the UV, you have to paint those Ao shadows by hand and they're not likely to be as accurate and certainly won't be as easy to generate.

Texture baking is something that would be excellent to have from within Poser. It would be a real game-changer and Material Room scripts and such would take off in a big way. But, it would also heavily impact the third-party re-texture market, since one could create a wide variety of custom texture packs relatively easily with a little bit of node knowledge. (Wouldn't take care of all concerns, however, but would go pretty far.)

PS - One can do texture baking from within Blender, which is free, and the process doesn't look too difficult for simple textures or something like AO and the like.