If they're in trenches, they aren't exposed. If they're not exposed, they can't shoot, and there's no point in firing up their engines. If they don't shoot, and they don't fire up their engines, they might not be noticed. If they're not noticed, there has got to be at least a .01% chance they won't be turned into smoking charnel houses, the interior of which are coated with a thin, greasy paste that once was their crew.
Sound to me like they've learned a lot. *grin*