oh, that's not hard to understand. i just wouldn't suggest it, for exactly the difficulty you're running into here. imho, you shouldn't have to rely on an image editor like that all the time. the only thing rendering everything separately should give you is either
- specialized lighting, which will break the coherence of your image if you have it in several different elements. 3d solution: optimize your figures' materials.
- ability to render everything, which generally seems to indicate a failing of the rendering application in handling resources efficiently. 3d solution: switch rendering apps, upgrade machine, and lots of annoying stuff. but, imho, the renderer should just be more efficient. and there are a lot of free renderers out there. it would be much more trouble at first to figure out how to get from Poser to something that could use them, but if Firefly really isn't working for you, switching is probably worth it.
- ability to move and scale around elements in the image editor, which should be done in the 3d app so that everything actually works together realistically. 3d solution: spend more time posing, working the camera and composing elements.
- ability to paint additions to lone elements. i've run into this a few different times, and it still wasn't awfully hard to deal with the one image. some masks (especially made with vector tools), some clone tool use, and some plain old time spent painting have usually taken care of it. 3d solution: render the few elements you need to be independent as silhouettes to generate masks.
i can't think of any other benefits off the top of my head, but have you got another? not that rendering to layers couldn't be useful, but using it on every figure in every picture seems to me like you're probably avoiding a problem that could have a more thorough solution. if you look at my gallery, you'lll see varying levels of postwork and overall heavy Photoshop use, so it's not as if i'm a 3d purist or uncomfortable with 2d editing. perhaps i'm missing an obvious benefit.