Originally Posted by sdsearch
Probably not much of a difference, because probably the vast majority of the passengers who do not accrue miles (but can*) would not accrue enough miles to matter before they expired.
Remember, at most airlines miles expire with zero activity in one to three years. A huge chunk of passengers who don't accrue miles are infrequent travelers who furthermore fly a different airline each time (because they choose based on price or convenient times, and may be flying to a different destination each time served most conveniently by a different airline for each such destination) and thus are not likely to repeat the same airline even every three years.
My gut feeling is that many people fly at least once every 1-3 years, often with the same airline (or a partner), and still don't bother. Furthermore, I noticed that a lot of travelers are not familiar with the notion of airline partners. I recently met folks who traveled with an RTW ticket, not realizing that all the carriers were partners in the same FFP...
Airlines publish revenue-passenger-miles (RPMs), but how many of these are accrued?
For example, UA
announced 9,811,660,000 RPMs in April '06, so potertially, if every passenger accrued miles on every flight, that would amount to at least this amount of miles (not taking into account bonus miles such as class of service miles, status miles, and special promotions) - or roughly the equivalent 400,000 US domestic Saver Awards... and that's just in one month.