Thanks for all the replies. I guess I just figured that covers for engines would always be sitting around somewhere, or would be on the aircraft itself, given that there always seems to be a set of chocks available to block the wheels.
Maybe I've been influenced by seeing military aircraft, which always seem to have all sorts of covers in place while they are parked.
Where I was born, dust storms are common, and I should think it would be a good idea to put covers on engines just to prevent dust from whistling through the engine, even if it's not volcanic ash.
I also recall reading that it's not a good idea to let jet engines windmill on the ground, since apparently that puts wear on the engine if there's no oil pressure to lubricate it. If this is true, it makes me wonder about what happens if you have to shut down an engine in flight, since it will be windmilling at a good speed for quite a while.