My comment about Air France was refering to the fact that I could easily fly Air France to PRG, and still earn the same number of miles, since I can't get the promo's. Instead I decide to give NW/KLM my business and I don't have any incentive to fly them, versus other codeshare partner (AF).
That said, my reasons for taking NW versus AF include:
1) 4 flights on DTW-AMS, providing my with better flight times (like the 9:30pm flight from DTW, or being able to arrive back in DTW before 1pm
2) Connecting in AMS versus CDG
3) Supporting my hometown airline - NW