I agree, I've taken out screens on intake pipes due to frost build up.
Well I've taken rodents, birds, and bugs out of intake and exhaust pipes because of no screens so I guess we cancel each other out. Anyways, they are required by code in my area, as is the purple pvc cleaner I had previously mentioned.
I was actually wondering about screens or protection of some sort, after the install. I looked at a few others in adjacent houses, none of which had screens either, so I just figured "eh, I guess that's how it is!" lol