I've been riding a trike now since the end of 2010. I used to post in the C&A forum when I first got my left knee replaced and got a 'comfort bike'. Riding, and riding itself is the key point I think. Whether you ride a 'DF', a recumbent bike, or a recumbent trike is more a matter of what you want and are capable of doing.
Most of the folks I know that ride a trike are more into the comfort and enjoyment factor than about speed. Most average people will be 2-3mph slower on a trike than on a bike, if that matters, then a trike may not be for you.
At this point, both knees, right hip have been replaced, and both shoulders have been rebuilt. Not needing to be too concerned about a fall, riding as slow as I wish to (lowest gear is 7GI with my Mountain Drive), carrying or towing gear when camping is a non-issue, and all day comfort with a great view (and a great place to sit when I stop) all add to my enjoyment of my trike.
I think the better question is "Why not a trike?" - And to answer that I would talk about speed and traveling issues that trikes *can* create.
FYI, BROL has an entire forum section just for trikes -
Trikes - BentRider Online Forums