I guess this is how you end up with well over a million BIS miles - but all on different programs. I change my program when it is advantageous to do so, and generally fly that airline as my preferred carrier. Here's my sequence for the past 25 years or so (by primary carrier):

AA -> TWA -> NWA -> UA -> BA -> UA -> AC -> TK

Wondering if FT'ers are more loyal or less loyal. Early reasons for changing were primarily due to where I lived at the time, later were mostly due to program changes