From my experience with my late DH, who had IV rehydration many times, the advantage with getting a line in and doing IV is that its much easier to add other drugs and so on through an IV line than getting a sub cut needle in for rehydration and then having to put an IV line in as well to administer other things. I can't imagine that there would be many times when all the patient who was that frail would need would be IV fluid without any other treatment and if they fail oral rehydration, there would likely be problems getting them to take drugs orally.
I couldn't find any research though.