They do get dusty, but I find that as long as I don't clean them I don't notice it. Just like I no longer notice having glasses at all, despite the opaque frames and the sharp boundary between clear and fuzzy vision, the dust just fades from my perception.
Same goes for computer screens:
This is great: instead of spending time cleaning my glases, and being frustrated when they're not perfect, I just wear them as they are and am happy.
Smudges, however, do need cleaning. While vision can easily work around the dust, blurry patches lose enough information that you can't subconsciously adjust anymore. I end up cleaning my glasses about twice a month when they get fingerprints or other oils on them.