sure if you are getting to where you can stand and walk a little bit after an accedent or devastating illness or the weather is just so nasty outside you can not possibly get on your bike and ride
I have really enjoyed spinning. I started out spinning and am going to progress to road cycling this month, so I hope what it has done is gotten me ready for the road.
As with all things the answer is it depends. My personal opinion is that Spinning can be a great asset to your cycling depending very much on the instructor and what you put in.
For example where I attend Spinning the instructors are high level, experienced cyclists themselves (Road & MTB) and bring to the class their expertise. These folk put together multiple sessions focussing on different aspects e.g. hill climbing, power output and endurance etc. Spinning never totally reproduces the road experience but can be used as an adjunct to develop and practice techniques to apply, particularly posture.
Spinning can also be used where you have limited training time available and to bridge bad weather or keep a baseline level of cycling fitness. We did some calcs using spinning bikes that showed power output and a 45 min full on spinning session by a qualified/motivated instructor equated to about a 80-90 min session out on the road.
What you have to look out for is instructors who are just going through the motions, this often gives people a bad impression of Spinning classes. If your considering taking up some classes ask about the instructors and talk to them about what you want to get out of the sessions.
You also see the folks in the class who don't have a good work ethic i.e don't turn the dial up or push out of their comfort zone, in these cases they won't get the benfits to apply to their cycling.
Anyway those are my thoughts
Cheers
Cy(cle)borg
P.S. I am not a Spin instructor just a very average (mainly road ) cyclist.