Yeah, great rationalization! This point largely devalues your argument in my opinion.

I am generally in favor of playing by the rules, and believe that fraud is wrong in and of itself.

However, two people essentially sharing the same frequent-flier account seems pretty harmless. I suppose that if everyone pooled their accounts, there would be more elite members, diluting the benefits for true frequent fliers -- but since only one person can fly and use the account at a time, is the system really any worse off if two people collectively fly X miles and earn and use the elite benefits in a single account one at a time, versus one person flying X miles and earning and using the elite benefits (or two people each flying 0.5X miles and crediting to his individual account)?