You get out what you put in. On training road rides, are you doing max effort? I think that's where spinning helps. Spin classes seem to keep the motivation up and drive you to work harder that you typically would on a solo ride. Of course this all depends on the individual.

Personally, I've only been to a handful of spin classes. I don't train indoors either since NCal has 4 season riding. The best thing about spin classes, for me, is to people watch.