That miles-earning card that earned you a free flight to Bermuda on your honeymoon five years ago isn't going to offer you such valuable perks anymore. In fact, it is probably costing you more to earn less.

Airfare cards are great for the sign-up bonuses, but for most travelers, that s where it ends, says George Hobica, founder of fare-tracking site AirFareWatchdog.com.

At the same time that struggling card issuers have jacked up interest rates and made it more difficult to earn rewards, their partner airlines have introduced tiered redemptions and fees that make those miles less valuable. It s a double whammy for consumers, says Curtis Arnold, founder of card-comparison site CardRatings.com.

Nevertheless, card issuers are aggressively marketing miles cards, primarily because they tend to be incredibly profitable. In the first four months of 2009, 30% of mailed credit-card offers were for miles-based programs, compared with less than 20% last year, according to research by Mintel Comperemedia, a financial services research and consulting firm. And as Delta (DAL) and Northwest prepare to merge this fall, American Express (AXP) (which issues the Delta SkyMiles card) and U.S. Bancorp (USB) (which handled Northwest s card and now offers the broader FlexPerks Travel Rewards Visa (V) card) have bombarded some one million frequent fliers with materials touting the benefits of each.

While it can't hurt to become a frequent flier in an airlines free miles program, consumers should probably send those miles-earning credit card offers through the shredder. Here are four reasons why:

Annual fees

Airline credit cards tend to carry an annual fee, which means frequent fliers must also be big spenders to come out ahead, says Avi Karnani, founder of financial management program Thrive. While these fees have been around for years, issuers may be less willing to waive them in the tight economy. The Continental (CAL) Airlines World MasterCard (MC), for example, carries an annual fee of $85, and earns one mile per $1 spent. One mile is worth about a penny at redemption, so you'll have to spend $8,500 just to offset the fee.

Constantly-changing redemption values

Odds are good that your miles will be worth less by the time you re able to redeem them. Both credit-card issuers and airlines can make changes that affect the value of your miles -- and they are, says Arnold.

Earlier this year, Citibank (C) changed its system, which offered flights valued at $400 or less for 20,000 points, to one charging 100 points per dollar, meaning that $400 ticket will now cost you twice as much. (For more issuer reward-program tweaks, see our story On the airline side of things, Delta changed its reward structure in September so that consumers can get reward tickets on more flights -- but they ll pay dearly for the privilege. At one point, a round-trip domestic fare would cost you a flat rate of 25,000 miles. Now, there's a new tiered system which offers redemption options at 25,000, 40,000 and 60,000 miles -- and makes it that much more difficult to snag a 25,000 seat. And, in many cases, travelers will have to spend more miles for the higher redemption option to snag a seat on the flight they want.

Miles still aren t easy -- or cheap -- to redeem

Once you ve got enough, it s harder to spend the miles, says Hobica. Travelers almost always have to book 330 days out when airlines first release seats for any hope of snagging a reward seat at the cheapest redemption levels. But now some airlines also charge fees for booking reward travel on a partner airline ($25 on Alaska Airlines), for last-minute redemption ($75 to $100 on United (UAUA) ) or for upgrading a seat ($50 to $350 on American (AMR) ).

Tickets are cheaper through other means

Most airlines require at least 25,000 miles for a domestic, round-trip reward ticket, no matter whether you would have paid $200 or $2,000 for the same airfare on your own, says Karnani. Folks trading miles for domestic, economy-class tickets are less likely to come out ahead of road warriors booking international fares or business-class seats. Most travelers will be better served with a cash-back card, whose proceeds can be used for any ticket or any other purchase you want to make. (You ll also knock out the hassle of attempting to book a reward seat.)