http://www.bcentral.com/articles/elliott/122.asp

7 Reasons to toss your ff cards.
1. Waste of time
2. Hard to get an award
3. Worthless (a mile is merely a penny)
4. Tax Liability
5. Makes you waste money.
6. Makes you make stupid decisions
7. Non existant perks

I don't agree with the article but some points have some merit, especially #2.

#7 is becoming more true, but the million mile a year flyer will get some perks. I've lost interest in DL because the program has gotten worse and don't try to add miles unless I have to. Other programs are almost as guilty