Another reason to spin: Interval training. I am a fairly new road rider (3 group rides now, I think), but I can tell you already that it is much more steady state riding than intervals.

Also, for me, spinning is just plain fun. I won't forsake spinning for road riding any time soon. Although I am planning on doing road rides during the week (before it gets too hot), when it stays lighter, I will still be going to my 3 weekly spin classes.