With the rumored Delta (NYSE: DAL) / Northwest (NYSE: NWA) merger hanging fire [subscription required], customers of both companies may well wonder what the impact will be on their hard-earned frequent flyer miles. The answer seems to be both good and bad.

The good side is that traditionally when airlines have merged, the frequent flier miles were carried over, allowing customers to select among an increased number of flights and destinations. There is no reason to believe this merger will be any different.

However, as travel guru Peter Greenberg cautions in his blog, airlines merge in part to take advantage of more efficient operations; i.e., fuller flights. Fuller flights mean a diminution of available seats for frequent flyers. He suggests, and I concur, that you not wait to redeem what you can.

I also suspect it's not a coincidence that United is going to start charging for a second checked bag, a trend I expect to quickly become the industry standard. Might airlines next allow customers to redeem frequent flyer miles to cover baggage costs in order to sop up frequent flyer miles at a bargain price??