Firstly I have to ask: "What is the advantage of having a long hose on your secondary, if you are giving the other diver your primary???"
There isn't any, and I don't think anybody has suggested putting a long hose on the secondary, if you donate the primary. What we have suggested is that anybody who is planning to donate the primary should have a longer hose than the 24" standard, because it's frankly miserable to share gas on that short a hose. And who would? The almost 50% of students who graduate from OW classes at our LDS and elect to purchase an integrated octo-inflator, but DON'T change the hose on the primary reg.
So, donating the primary ISN'T a tech/cave notion, but one which is as common as donating the octo, at least at my LDS (which is NOT a DIR-sympathetic one, but simply standard recreational). Then the question is what hose length you want to donate, and some of it comes down to routing. A 36 or 40 inch hose on the primary is a PITA. You can route it under your arm, but then it comes up to your mouth at an acute angle, and you have to add a swivel or angle adapter to be comfortable. If you let it float out around you, it's a huge loop of hose looking to get tangled in anything you come near.
The 5' hose wraps neatly around the body, leaving no large loops, and comes to the mouth at a comfortable angle. It permits donation of part of it (about the same as a 36 inch hose) or all of it, if everybody's relaxed and swimming is necessary (or to do a comfortable ascent). The problem with the 5' hose is that, for people with larger frames, it's too short. At that point, a 7' hose is a better choice -- STILL permitting partial deployment. But I will agree that a 7' hose can be difficult to route with a traditional BC. I never had much luck getting one to stay under a waist belt, and the setup is designed to route under a light canister, pocket or knife at the right hip.