I'm going to disagree with the idea that this would be a bad material for a sword, with the prevalent idea that it reduces the force of the swing. The force of a slash is determined by more than just the weight of the weapon, it's also determined by the speed. In fact, it's determined more by speed than by weight.
Newton's second law of motion states that F = ma, also written as $F = \frac{1}{2}mv².$
https://en.wikipedia.org/wiki/Force#Second_law
https://en.wikipedia.org/wiki/Kinetic_energy
Since we are not changing gravitational constants, anything we do to the weight will have the same factor of change to it's mass.
If we reduce the mass of a sword by a factor of ten, and keep speed or acceleration the same, then yes, the force is also reduced by a factor of ten. However, a human using a much lighter blade is going to be able to move that sword much faster than the heavier one. If we make an oversimplified assumption that if we reduce the weight by 10x, then we can increase the speed it is moving by 10x.
Being generic, we now have the formula of $F = (1/2)(m/10)(10*v)².$ Doing simple algebra, we can reduce this:
$F = (1/2)(m/10)100v² $
$F = (1/2)m(10)v² $
$F = 5mv² $
So, this says we've increased the force of the blade by 10x by reducing the weight 10x.
If we only triple the speed at which we swing the sword, we can see that we have almost the same force of impact:
$F = (1/2)(m/10)(3*v)² $
$F = (1/2)(m/10)9v² $
$F = (1/2)(9/10)mv² $
$F = (9/(2*10))mv² $
$F = (9/20)mv² $
$F = 0.45mv² $
Now consider that if a soldier trains with a regular steel sword, then uses the lighter version in battle, they will have just as much muscle mass as the soldiers with the heavier sword. The lighter sword will be so much easier to swing that they will get a much faster slash than with a heavier sword. This adds the ability to change the angle of attack faster as well as simply attacking faster, and attacking for longer stretches of time.
A lighter sword to carry over their forced marches would make them less exhausted or able to travel farther. I carried an M16A2 during marches in Army Basic Training, and at 8.8 lbs, it gets really heavy quite quickly.
JollyJoker, in a comment on another answer, says that a claymore weighs around 5.5 lbs. A katana weighs around 2.5lbs, even made from steel. I think you would find fighting styles closer to a katana with the 10x lighter swords. You might also find fighting styles closer to kung-fu, which also uses lighter swords.
https://en.wikipedia.org/wiki/Katana
Another answer suggests fighting styles along the lines of an epee/foil/rapier, but if the enemy is using armor, that's not likely to work. You would have to find the joints of the armor to attack, and that's pretty hard when the armor is constantly moving. Unless the soldier is an expert fencer, they would basically have to be lucky or wear their opponent down in order to dispatch them.
A TV show I saw a while ago (BBC or a Nat. Geo.), showed that a katana is actually better at attacking armor than a traditional English sword. They did actual tests with actual armor and swords.
The long sword put a decent dent in the armor, but the katana cut into it. Unfortunately, the person doing the tests wasn't really trained in either style of sword, and I don't think the swords they used were of good quality, but you get the idea. (A trained samurai would aim to use a spot about 1/3rd to 1/4 of the length of the blade from the tip, not the center of the blade, like in the video.)
https://www.youtube.com/watch?v=EDkoj932YFo