Syn's always helpful...racking up serious good karma points here.
Plastic bushings will last a long long time....at low power and without extended (i.e. 3000-4000rnd) bursts. The problem is that once they wear, they allow the gears to slip out of alignment and then much worse things can happen. If the gears slip and "crash" then you'll end up with a broken gear set (~$55-90), a mashed piston ($15-20) and maybe some other parts... (or you have really bad luck and toast all the parts at once...
)
So, that's why you hear so often about people switching to metal bushings (not bearings) when they open up their mechbox. For the cost of them it's just so convenient and easy to do while you're into the guts of your rifle. You do need to re-shim things...and a good shim-job makes or breaks an AEG.
Hope that helps,
Tys