This "wear and tear" that everyone speaks about is very minimal, if not, non-existant. The spinning device doesn't run on gears like a car engine. It runs on an
electric motor. That means there are
no connecting parts between the spindle and the motor. There are probably just ball bearings or fluid to reduce viscosity. This also means that the GC has no brake pads either. The motor itself acts as the brake by stopping it's pole oscillation. That's why the disc keeps spinning when you turn off the power. When the motor is off, there's nothing to stop the spindle from spinning.
So, actually the opposite is true. It's more likely a good thing to see that the disc keeps spinning when the GC is turned off. It shows that there is very little friction between the spindle and the ball bearings/fluid. If it were to stop immediately after powering off, I'd be worried that the ball bearings are broken or somehow fused to the spindle.
Oh, and Tsunekazu's answer is the more likely reason for the continual disc spinning. Pop a CD-ROM in your PC and see how long it takes to access data from it. Now imagine if your GC had to do the same thing while you're playing...
If anything in the system were to have "wear and tear", it would most likely be the guiding track that the laser rides on. That thing is prone to misalignment.