We are! Some cards you can get the first year for no fee, like the NWA credit card. So take the card, get the miles and any other perks, and then cancel before the year is up.

Other cards, like Delta's, give you elite miles, so if it gives you enough to make the next tier and the annual fee is LESS than the price for a ticket that would give the same elite miles...then why not, plus you save the time it would take to do the trip...