Plugins work well if the system is designed well from the beginning. Most software developed today uses some form of components. If you have an accounting program and need some spreadsheet functionality, you probably buy a spreadsheet plugin, not write the code from scratch into the program. With a good base architecture, only a major change should necessitate upgrading existing components. A lot of small companies simply don't have experts in every area. Poser exports RIB files. If you can find a really great Renderman renderer why try to do your own, make it a plugin. Photoshop certainly doesn't seem to have suffered from having a plugin architecture. Indeed, it's so successful that Corel and others have adopted the standard, making more, better and less expensive tools available for everyone. If you start out determined to support a lot of legacy code and then try to add plugins on top of it, then the chances for problems are increased.

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