With which product sources does this problem most reside with?  I can understand freebies from ShareCG or similar being all over the board (like no runtime at all) since there is no QA process to get something published (that I know of), but doesn't Renderosity and Daz have a fairly stringent QA that could be leveraged to fix this?

With the exception of the common, most widely used packages, I generally build a fresh "project-specific" runtime whenever I start a new project.  That way I'm only presented with what I need and I can contain the "mess".  I also maintain the original product download in a very granular folder hierarchy that makes locating them much easier.  I'll also stick original zips or exe's in another archive named to match the exact product name and include some product shots for easy reference.   

I know this strategy wouldn't work for everyone but since I need to be able to account for every dollar spent, it works.  By the way, this problem isn't just a Poser problem. :)  It would seem that organization and art mix about as well as initiative and complacency.