spinning means:
1) less lactic acid build up which means both aerobic and anaerobic system work more efficiently and less burning pain
2) less use of muscle glycogen which means that when you do need that big burst of energy you can do it.. for racing, when that attack comes at 100km you can go with it.
but there is no free lunch... you are off loading work from your anaerobic system to your aerobic system (heart and lungs).. but as long as blood sugar levels are kept sufficiently high (you keep eating) your aerobic system can keep going basically forever... when your muscle glycogen is gone it takes long time for it to be replenished... even your aerobic system functions better in the presence of lots of muscle glycogen. you do need a good aerobic system though to sustain your cadence to get the same power output
spinning is actually less 'efficient' than "mashing" since your legs must move themselves (their weight/inertia) more in a given amount of time... so you should actually use more calories (may need to eat and drink more). just like you are going to use more gas driving on the highway in 1st gear rather than in 5th and why engine brakes work etc... but that being said, cycling fast (or car racing) is not about being efficient it's about getting from point A to point B the fastest. it's quite individual but given a person's physiology, "spinning" has a lot of advantages over "mashing"