The difference between business class and economy is often not that great on flights within the U.S. or Europe. But on longer flights the difference between the back of the plane and business class may make the difference between arriving fresh and ready to go, and being a complete zombie for a few days.

Which makes offers like American Express’s Platinum program and British Airways’ “two for one” Visa Signature card so enticing. Buy one, get one free? What’s not to like? As it turns out, plenty.

The latest British Airways offer sounds like a great deal, available only to holders of Chase BA Signature Visa cardholders. A companion ticket anywhere British Airways flies from the United States, in either First, Club (business), or World Traveller Plus (premium economy) class.

But is it a savings? Well usually, not exactly.

There are plenty of simple conditions, as in the two passengers must travel together for the entire trip. Although this can be a problem, if say a spouse or friend wants to come along for only part of a business trip.

The real catch, however, is the part that says “non-restricted, fully flexible, full airfares.”

Here are a couple examples: A British Airways San Francisco business class ticket roundtrip to London is currently is $14,911. Which includes about $618 tax, which must also be paid on the free ticket. But the lowest discount fare for the same flights with 14 days in advance and the same weekend stay requirement is $3219.

In World Traveller Plus, from Chicago to Milan, the unrestricted roundtrip fare is $4909. The same flights can be obtained with only seven days advance, for $1861.

Of course, all of these fares are subject to change, but the general principle applies to other British Airways fares, and to other airlines offering this kind of “deal.”

One situation where passengers might personally benefit is if their corporations or a client would be reimbursing them for one ticket. But it’s hard to see how the corporations themselves benefit.

In fact, even at the last minute, when a real sale might not be available, there are almost always cheaper tickets available than the unrestricted full fare.

In British Airways’ case, the offer was scheduled to expire but has been extended. So it’s hard to know if British Airways has been doing well with the promotion, or whether they are extending it in hopes of getting more travelers to take them up on it.

photo by garybembridge on flickr.