One advantage of having a long massless sword is that it would be a lot more agile than a normal heavy sword of the same length.
A story I remember involved the crusades, where a knight would go in with his big, heavy sword, swing at a Turk who would deflect it with his much lighter and faster scimitar, and then get inside the swing to attack.
Another advantage is that with enough mana there isn't an upper limit on length, so using it against cavalry would be a lot better. Especially if you could dissolve the blade and then reform it, and not have to worry about pulling it from an enemy or having it yanked from your hands.
The "blade" would never get dull, and there doesn't have to be a limit on how thin the blade is. This lets you have a blade thickness that approaches zero, meaning you can have something like a monomolecular wire. Something that thin would pretty much ignore any armor, flesh, and bone that it passed through, providing an infinitely sharp cutting edge.
Lastly, the blade could have a variable shape, so you could form a hook to pull away shields or yank away swords, then straight to slice and dice, and then wide to block arrows.
It could all happen faster than an opponent would be able to react to.