Well, all I can tell you is my experience with a K&N on my 99' 400EX quad. Ran a K&N for several years w/ an outerwear. Always kept the filter clean and oiled.
I recently rebuilt that engine because of a failed re-ground camshaft. The cylinder showed the telltale scratches caused by dust and other particulates getting sucked into the cylinder. My (very experienced) machinest pointed that out to me. It was pretty clear. He didn't know that I had been running a K&N. Sometime later, I overheard a conversation this machinest had with another customer about K&Ns and he said he can usually tell if a K&N was being used just by looking at the cylinder walls. Keep in mind, we're in the Phoenix metro area. Sometimes we'll have dust storms instead of rain storms.
Now, had the camshaft not failed catastrophically I would have had no idea about the internal wear in my engine. The cylinder wear was not keeping the engine from operating at a (seemingly) normal output. Nonetheless, the cylinder WAS wearing due to particluates being sucked in.
I used K&Ns on my streetbikes for years and would again. I don't frequently ride through clouds of dust on a streetbike. I do frequently ride through clouds of dust on my OHV machines (mostly deserts and The Dunes) so I choose not to use K&Ns in those conditions.
That's been by experience. Take it for what it's worth...
The dirt that caused your camshaft scoring was in your oil (or lack of oil). If you have enough dirt in your oil to score the cam, there's bound to be enough to ruin the cylinder also. I live in AZ. Dusty all the time. MY experience with K&N's are as follows (besides the XL250), Volvo 240-wrecked with 250K miles, Chevy S10-wrecked with 204k miles, current Volvo 850 has 175K miles, my C&J XL440 ran for 19 years with one rebuild. I roadraced it, MXed it, commuted on it and even rode it to Alaska. To me, the proof is in the pudding.
The "you can see through it" doesn't hold weight. When I compare the inside of these filters with the dirty outside, it's obvious they are working.