The more you work out, the more calories you burn, so you will obviously see a benefit there. Whether an extra spinning class will add anything to your overall fitness, however, is harder to determine. Your body does need time for rest and recovery to get stronger. If you add the class in, but start to feel sluggish or fatigued, then I'd suggest that you might be overdoing it. Either cut back down or use the extra class as a "recovery ride" by keeping the resistance low and your heartrate down.
Whether an extra class will make make you stronger or contribute further to your cardiovascular fitness depends, in my opinion, on how your classes are structured and how you're riding them. Just as doing endless miles at Zone 2 on a road bike won't necessarily make you stronger muscularly or aerobically (although it will likely help with endurance), neither will riding more miles on a spin bike. I would suggest talking to a trainer at your gym to determine how to approach your spin classes so that you can best meet your fitness goals.
I'd also suggest that you might benefit more from adding something into your routine, either in addition to or in lieu of an extra spin class, that develops core strength, flexibility and balance. Yoga and pilates obviously come to mind.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher