Minor maim debuff applies a 15% reduction in damage to enemies. So a handy debuff for group content with a large amount of mobs or players if you can apply it to a large group for a reasonable cost.
DKs have a good AoE minor maim skill with choking talons. NBs have a good AoE minor maim skill in PvP with Mass Hysteria not so much in PvE where you want to keep the mobs together.
Everyone has access to minor maim for one mob/player with slash one hand skill. NBs also have shades which apply it on the mob/player being attacked. So why consider Knightmare set?
Knightmare set provides a chance of an AoE minor maim. For tanks it procs regularly and provides a good group benefit debuff. The other benefit is that unlike most tank sets Knightmare has unique rings that provide stam instead of health for the rings in Spindle dungeons. The debuff does apply to bosses as well it seems based on the animation.
So while it won't have a lot of benefit for DK tanks running choking talons or NBs running Mass Hysteria in PvP the set does work well for the other classes when tanking group content. It is also easy to farm since it is not a popular set, appears in the golden vendor and is in a tank friendly dungeon Spindleclutch 2 (can you ever have enough bloodspawn?).
PUG Life - the true test of your skill
18 characters, 17 max level, at least 1 Stam and 1 Mag of every class, 1 of every race and 1200+ CP
Tanked to Undaunted 9+ Mag and Stam of every class using Group Finder for 90+% of the Vet Dungeon runs