Are air filters a bad idea?

[glenn in camino]

I just put a K&N air filter attached with a C shaped PVC pipe on my A carburetor. I can't notice any difference in the way it runs but I've heard that air filters are a bad idea. Please explain. Out here in the country many of our tours encounter dirt roads. I've also installed an Afordable oil filter. I want this engine to last a long time.