"recently ive been setting mine to the earliest version of poser known to support the features used in the product. im not going to set it back to, say, P4 if the product only works in P8+."

That seems like the most reasonable approach to me. 

 I wouldn't expect vendors to supply .rsr or .bum files. OTOH Having the application recognize .rsr files and convert them silently to .png is just a nice thing to have, given there's still a huge amount of 'legacy' content out there (and sitting on our HDs) that is still quite usable. I can understand taking it out of the main program as it evolved but they could stick it in an external library and call it when needed. It shouldn't be difficult to implement, and I say that only because I wrote a rsr2png.dll  Might be even easier for the Mac since .rsr is just a Mac .pict image file with some additional stuff tacked on. It's not a huge deal I suppose, just a matter of avoiding annoyance/work for the user if it can be done without a great deal of work or sacrificing performance etc. 

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken