I think I have read they were made that way so they are uncomfortable if down to "encourage" you to raise it. Most people don't realize a head rest is a safety thing to prevent your head snapping backwards in a crash, not just a place to put your head. The back row is tricky because the headrests have to be kind of low and hidey so you can fold the seat, and I'd guess 9 times out of 10 if you put an adult back there they would never think to raise it. The Odyssey gently "reminds" them.
I agree they look WEIRD if all the way up and I wish it wasn't all up or all down but I am used to having 2 things back there - I have two kids in tall car seats on either side - so I have learned to work around that. You use your side view mirrors, you check a little more carefully for "invisible" cars, etc.
I have to admit when we first put the girls in the 3rd row like that in the old Sienna... I backed into a car in a parking lot. There was a tow truck parked right behind me. So I looked right and left and right and no one was coming and then I backed into a tow truck. I was mortified. It was not one of my finer moments. So I've really learned to work around things in the back seat since then.
The back up camera in the odyssey is probably the single most thing that gives me any reason to say it is better than my old car. Of course that isn't going to help when you are driving forwards... and it's not in all models now but I think it will be son - but you do learn to work around things like headrests when changing lanes or whatever.
The above tip might be useful - mine are just removed and in my basement because you can't install a car seat over them. of course then if you are out and need them, you don't have them.