Some swords in actual real-life use were so long that they had to be carried in an over-the-shoulder rig: nodachi (like the big sword carried by the low class guy pretending to be a samurai in the fairly accurate Seven Samurai) and just about all of the two-handed swords in the late middle-ages and Reniassance. There’s no way to carry a sword that big other than over your shoulder or on a horse. Usually that wouldn’t have been your only weapon, and those big-assed swords were usually only used in military situations, so you’re right that, in general, that kind of thing isn’t very practical.
To be workable, back sheaths for regular-to-long blades can’t really cover the whole blade. If there is a slot in the side of the sheath, you can actually draw a long blade from over your shoulder. It’s not fast, but it lets you get to the blade without having to get out of the whole harness. As far as I know, no one can prove without a doubt that such rigs actually existed, but there is some evidence that they did. Chinese martial-arts masters, whether appearing in a movie or not, probably use (would have used) something similar.
My own youthful experiments with a (zinc) katana convinced me that anything close to full length blades can’t be carried cross-back with a full sheath unless you have arms like a gibbon. I actually do have longer than normal arms and I can’t pull a normal blade length for a Japanese from a cross-back sheath (i.e.: point at left hip, hilt at right shoulder, right hand draw). When I hung it on the left shoulder, hanging straight down, however, I could just clear the throat of the saya without stretching. Just for fun, I tried the same thing with my iaitô a couple of months ago and even though the blade is longer than average for Japanese katana (average is 2 shaku 4 sun, mine is 2 shaku 5 sun 5 bu), so that it fits my body proportions, I can still draw it if I carry it slung over one shoulder. Realistically, I’d just shrug it off my shoulder, catch it in my left hand, draw it, and drop the saya if I were in a hurry. Then again, I wouldn’t be carrying it over my shoulder if I were interested in getting to it in a hurry.
Rolls with a sword on your back are just about impossible, by the way. Believe me, when I was a stupid 13 year old I tried just about everything to get it to work. The sword falls out; you can only roll in one direction, opposite the shoulder the hilt is on; if the blade is lower than your hip you can’t roll at all in either direction; and it bloody well hurts to roll over the scabbard. Plus, if the harness is tight enough to feel secure, you’d be lucky enough to be able to squat, much less bend your back enough to do a decent roll. Ninja super-bounce-ball gymnastics would seem to be unworkable in real life, not that that’s any surprise. You can roll if you carry a sword at your hip, like normal, though it takes some practice to avoid tangling yourself up.
Some fantasy blades are both cool looking and practical. Most of the Lord of the Rings blades are very nice. I especially like the elven swords like Arwen’s, Strider’s original sword, and Sting.