I forget the exact poster and the specific thread, but a few months ago a DL fan pointed out one big benefit of DL that I had never before heard of: they apparently release a lot of award availability very close to the travel date and they have no near-in ticketing fees for elites. (Maybe mid-tier and higher...I don't know.)

So this poster was saying he'd use DL for last-minute getaways, whereas the *A/OW programs are more useful for bigger, more planned-out itineraries.

I've never held much DL so I can't really add personal experience. It's a niche strength I sometimes wish I had. My wife flies just enough DL to make holding the DL Amex worthwhile, and we've used DL miles for Central America at about 2cpm, which isn't too bad.

I still prefer UA for the raw number of choices and one-ways everywhere, AA as a 2nd choice for pretty decent availability and one-ways, WN for cheap awards for family, Avios for short-hauls, and US for *A partner awards....all of these are more usable to me than DL.