The BA and DL issues are different...although both are a pain.
BA has plenty of seats available (well, sometimes), but offers few if any "pure" awards on their own metal. Everything is "Cash & Points", usually a *lot* of cash, making their coach awards quite laughable indeed. Still, if you can redeem AA within the Americas, EI to Europe, or a couple of others, it can be a somewhat usable program. It can also be worthwhile to redeem on BA metal if you're doing the 2-for-1 First Class award that is a long-haul flight beyond LHR (e.g., South Africa or India).
DL, on the other hand, rarely has the seats to begin with. It's of such limited utility to me that anytime I see even the most mundane 25,000-mile award within the U.S., I'm tempted to grab it. Managed to get a simple MCI-EYW R/T two years ago and was thrilled about it. That's how low my expectations are of DL. From what I understand, those who would argue in favor of DL would point to last-minute award availability being fairly loose and without fees.
The round-trip requirement imposed by both DL and US significantly reduces the utility of those two programs IMHO. At least with BA, I can redeem short-haul one-way AA flights without too much difficulty.