This How To Clean Hydraulic Brake Pistons (Park Tool's Tech Tuesday #104) shows how dust particles can interfere with the proper functioning of hydraulic pistons.

Disc brakes have been adopted so often, one could almost suspect rim brakes will vanish. Likewise, it is becoming increasingly common to see hydraulic disc brakes thoughout line-ups of MTBs.

How do hydraulic disc brakes make sense on MTBs, given the tendency of pistons to "stick" when dust makes its way, as it inevitably will on trails?

In other words, cyclists who hear gritty noise from otherwise well-tuned rim or hydraulic-mechanical brakes can simply squeeze the levers a couple of times to crush the sand particles and keep going, but with hydraulic pistons it seems that getting rid of sand/dirt is an intricate operation, and it may be necessary to put up with extended noise from brake pads that became stuck near the rotor; is that accurate?