i would use it
except that i know of several computers that have met their deaths at the hands of unexpected liquids
Unfortunately, my buddies computer (back in the day) didn't survive the un-expected liquid even with a completely closed case, beer spilled onto his desk, ran down the table, hit the top of the computer, and ran down into the power supply and POP.. blew the circuit breaker too. the only survivors were the HDD and some ram.
And as for dust, i'm not so sure, mine stays pretty caked up regardless of open or not open case. speaking of which though it's about time to fire up the compressor and blow out all the cases.