As Concours posted, objective testing has been around for a long time re KN filters showing that they do not filter as well as paper. The issue becomes, as was noted earlier, do they filter "well enough?" For practical purposes in most situations, the answer is "it appears they do." Basically, for normal operation, the filter difference PROBABLY makes no practical difference in engine wear for the typical rider. The advantage of the KN is that it can be cleaned though it's not as if paper filers are expensive!
They can also be used in situations where, for whatever reason, the stock filter system is not wanted - maybe for appearance/clearance for modifications, etc. The CNW Norton E-starter is a good example; the OEM airbox will no longer fit; a KN will.
Rant follows...
The claims that KN or other aftermarket filter can flow "more air" and increase power is one of those marketing things that is based on several premises that almost never exist. Yes, a KN filter of the same surface area CAN flow more air than a paper filter. But does it matter? Not usually, unless the OEM air filter or airbox is, intentionally or unintentionally, undersized or poorly designed. If an engine needs, say, 450CFM of air at WOT to make max power and the paper air filter can provide that, the engine makes its max HP. Installing a filter that can flow "more air" does nothing because the engine can't use it/won't "pull" any more air.
We did tons of dyno tests years back on air filters and never found any HP to be gained on a stock engine by just installing a KN (or any other) aftermarket filter in place of a new OEM paper filter. I could tell you many stories (several of which I was directly involved in the dyno work to "support" the claim) of the way aftermarket manufacturers make HP improvement claims. In most cases it wasn't really "false advertising," it was advertising that left out some information...like, the 13 HP increase published in a magazine by a well-known (and very loud) aftermarket muffler was actually based on a comparison between a new, aftermarket muffler against an old, rotted out OEM muffler. In reality, on the dyne a new OEM muffler produced more HP than the aftermarket "performance" muffler and was quieter as well.
Then there was a well-known drag racer that used a famous additive in his Pro-Stock race car. It was obvious he used it (and that you should too!) because the ad, filmed at Thunder Valley, showed him pouring a bottle of it into the engine at the race track between rounds...except the additive wasn't in that bottle; the same oil that was used in the engine was in the bottle - FWIW, Lucas 0W5. (Yes, you read that right. ZERO W Five.)
Rant complete...
