The Martial Knowledge set places a stackable 8 percent damage taken debuff on enemies when you Light Attack them with less than 50 percent stamina remaining.
It has a 5 second duration on an 8 second cooldown, giving an effective max uptime of 5 percent damage taken. This does stack with Major and Minor Vulnerability.
This will be lower than the advertised amount, just due to how the mathematics work.
This set was nerfed during the first PTS cycle where it was revamped, from an effective 6.6 damage taken debuff to the current 5 percent debuff.
In terms of what role will be wearing this set, both tanks and healers could use this set.
I would highly recommend all support mains to pick up this set. It is an old set, dropped in Craglorn, so it should be fairly cheap to pick up. If all else fails, you can always try to get some drops from Skyreach while grinding up to max CP or helping a friend grind out levels!