Often considered the most essential class in every playthrough, due to their strong healing and buffing abilities. Master of self magic (spirit, mind, body) and mirrored magic (light, dark both in MM6, one in MM7, only light in MM8) Worst offensive fighter, but more durable than sorcerers and druids In MM6 you can use knights as your misc skill bank, since every class can master every misc skill, you can let the knight handle everything as he doesn't have to spend skill points on magic.
In MM7/8 they stay relevant all the time. Due to no armsmaster their powerlevel falls off in the late game of MM6. Powerful combatant, makes the early game much easier. MM8 skills are identical to MM7, with only two changes total (staff GM ability and regeneration spell) but the class structure changed and a few new skills were added. The ratings are given from 0-10, most skills are pretty much identical in 6-8, 7 adds the grandmaster rank, which doesn't really change the value of the skill itself, but some skills are noticably different between 6 and 7, which is why they get a different rating.