For me, and this is just the motivation for me making a purchase of an upgrade.
I upgrade SONAR precisely for the plugins. That is, there might not be enough feature additions (or better yet), feature updates that interests me and the plugins are the tipping point in my purchase. I'll be honest with everyone, there hasn't be a lot to get me excited about SONAR since the X series roll out. And I really want to be excited about the product like I once was.
I do the upgrade path hoping that as long as the lights are on at Cakewalk, maybe someday the little things that I would like to see happen will appear. Maybe I am using another product until that day but if I can get some plugins that will work in my other hosts, the purchase of the SONAR upgrade is still a positive in my mind.