I agree that it would not be a big problem normally. But, even when you tank is drained and dry, the water is still coming. Any moisture in the air is transmitted to the end of the air nozzle at about a 8:1 ratio. This means you are introducing 8 times the amount of moisture into the case that would have been there.
I also think that the bit of moisture that enters from this is not going to be a problem, especially if the engine will be run soon.
But the moisture and dirt are there, and immediate. Just hold your air nozzle close in front of a mirror while releasing the air full force for a few seconds. Now take a look at your mirror.
I am always concerned about the moisture, rust, and hydrocarbon content of compressed air, so my compressor is rigged up with a coalescing filtration system that provides me clean, dry air anytime I need it. It also automatically drains the tank.
Folks who do not have adequate filtration on their compressor (probably 99.9% of private compressor owners, and 90% of industrial ((believe it or not)), are going to be introducing iron oxide into their engine case. I guarantee this because your compressor tank is full of the stuff, and it sends it downstream whenever you release any air.
I didn't mean to open a can of worms here...but Ivan's worries are not quite as unfounded as one may think.