Heat is more likely to be the issue than dust. Unless all of your roof is in shade the attic will get very, very hot in the summer.

My servers live in the garage and they get all sorts of dust and crap through at them. I have never cleaned the insides of them in the 5 or so years they have been there and they are still going strong* (even though they also regularly see 80% humidity as well).

* ok, so the PSU fan on one makes a nasty noise and 2 of the 8 disks have failed/are failing, but they are still running