Over the years, I've often been asked why we don't use doublehanders on the coast in Scandinavia. Well, you could, but 99% of the fish wouldn't be much fun at all and they're not effective when you strip in to the leader (which some do, sone don't).
I've seen and fished with people using a traditional 14' rod in the salt, and to good effect, but they're weren't getting things done any easier than with a singlehander.
Then came the switch rods. For those who don't know, switch rods are shorter doublehanders (some say 1,5 handers) where say a 7-wt casts a line that rates somewhere between a single hand 7-wtw and a double hand 7-wt (which really confuses some). A few brands make their switch rods so they cast doublehand ratings, I think, but for the most, they're strange in-between-things.
I've had two, but never really used them - to me, they're exactly what I said above - strange in-between-things, where I prefer either all the benefits of the double hand rod, or all the benefits of the singhand rods. I know somw say that you get both in a switch rod, but I sort of disagree.
But - in the PoD you see my friend Niels. He's very happy with the advent of switch rods, becasue he's got a shoulder injury which means that he can't cast a single hander for more than an hour or so, before he has to give up.
But with the light double hander he can fish all day, and still enjoy the fishing, not casting and fishing gear too heavy to be meaningful.
So for some they certainly fill in a gap, make sense and maybe for me as well - someday (perhaps when I get old), and when that day comes, I'll have plenty of single handers to swithc(!) out for light double handers.