The full-fledged GIMP package, with all the plugins and stuff, takes awfully long to load, and most those aren't really usefull for the standard MS Windows plug-and-play user (which I believe to be the target audience for most of the stuff in PortableApps.com). So, why not release a lightweight GIMP package with only the essential plugins and stuff? I tweaked the installation in my pen-drive, and now its load time is pretty decent...
Just to make it clear, I'm not saying the full package should be dumped. I just think having two parallel versions (full/slow-loading and incomplete/not-so-slow-loading), in this specific case, is really worth while.