Jeff, I don't think in-line groundbait feeders are a gimmick.
I have been using in line maggot feeders with a two hook rig for a while now and with some success. I have been toying with the idea of using a longer than normal tube on a groundbait feeder and having a helicopter link coming off at this point in addition to the semi fixed bolt rig on a short link at the other end. This should give a good bolt effect and stop the helicopter link ending up being pulled down to the swimfeeder. I am intending to do a few tests to compare the hook up rate of different hook length materials and methods and two rigs fished within a few inches of each other should provide some interesting results.
However, read my sentence again "There are some variations now that have a tube fitted so that, like the in-line closed end feeder, they can be fished in-line, but many anglers regard these as no more than a gimmick."
It's what I hear. No doubt there might be a very odd occasion when some sort of rig like you say just might catch a fish or two, but you have always got to ask yourself, is there another, maybe better, way of presenting it that's tried and tested.
My point being that you can take anything used in fishing, add a little something to it (in this case a tube) and says it's "revolutionary".
Over the years there have been lots of little twists on standard bits of equipment all claiming to have "the edge", but few stand the test of time, sadly.