Uncle

At my B Service, 40K miles, the air filters were changed. At 53K miles, I inspected them, , brushed them off, and cleaned the inside of the housings. Frankly, provided they remain clean, why should I change them?

tiyyara

a Mann set is about 30 usd
if you took all the trouble to take the engine cover, air filter housing and the air filters out, why would you not change them.
a 1 mpg change in mileage will pay back 20 gallons over 10 thousand miles, which is about 80 USD
Even if you get the mpg improvement only for 5k miles you are still ahead

C300Sport

It's good to pull them out from time to time to clean any big stuff out of the housing, insure the filters are in good shape and maybe knock a bit of dirt off/out.
Always change them at the recommend service interval or sooner if you run in dusty conditions.... You can't always see the dirt that is impeding airflow.

acr2001

When you clean them you are only removing the big bits of dirt on them. Tiny particles become embedded in the filter, reducing it's air-flow, even when it otherwise looks clean (the filter will be a darker color and if you slap it against a wall, you'll see powder come out). Lastly, as the air filter gets older, the fibers loosen up and spread out, allowing more contaminants past the filter. Cleaning them can also speed up that process. Cleaning a filter will certainly improve air flow and make a difference, but I wouldn't go on doing that for too long.