It doesn't cost NW anything to reward its elites differently from DL or CO. They were already different from DL in giving top tier at 75,000 miles, and different from CO in giving 1.5 miles credit for higher fares.

So the airlines are varying it even a little more? So what? They are all pursuing their own ideas about the best elite (and cash) retention strategy.

Personally, I sure hope that NW has got it right, since I'm one of those mixed fare loyalists they are pursuing. I can be bribed. I will spend money with your airline if you give me an incentive. Thank you very much.