For what it's worth...I just installed X3c (and set a system restore point first, just in case). I opened a project and it had a
ton of missing plug-ins, including Cakewalk ones, which concerned me. I closed Sonar, opened, created a new project, and ran the VST scan. After the scan was done I opened up the original project, and all the plug-ins loaded properly.
Just to be ornery I rooted around my hard drive for some projects done in X1 and X2, and they all loaded properly too. This included several projects that had lots of plug-ins from Waves, Universal Audio, and other companies. So either I'm lucky, the Gods of Sonar like me, or the VST scan thing is indeed an important part of the update process.