It's not fatal but the little inconveniences add up: usually delta.com won't show availability and you will have to call. If you are elite with FB you get preferred award seats that you will not be able to get with your DL miles.

I once managed to get 100,000 skypesos and to burn them on a TATL business class return... on Delta. I would have preferred AFKL but no availability showed up on delta.com and I did not feel like dealing with the unhelpful human pods which populate airline call centers. DL's business turned out to be pretty good, the only issue is with their atrocious terminal and ground service at JFK.

Unless there is a bonanza of bonus skypesos to be grabbed, I'd rather get a regular AmEx charge card, or a Starwood AmEx, and convert the points to Flying Blue miles. Note there is a 25% bonus everytime you transfer 20,000 Starwood points.