It's not the disc starting and stopping that shortens the life. It's the thermal cycling - warming up when switched on, and cooling down when switched off - that causes degradation and eventually failure. For laptops, it's a pity about the battery, though. It depends on what proportion of the time you would normally want to have it switched on. For mains powered, there's not very much in it, and you can hear an argument either way, but I think the consensus is that less pollution is caused in generating the energy to keep it warm on standby (or ecosave, or whatever) than in manufacturing the new one that otherwise you'll have to buy a few years sooner.