One of the Defiler buffs not mentioned is a tank-buff that proc's DPS/Haste debuffs on mobs that hit him. Proc amount can be upgraded in the Defiler AA line. It's main use is seen on multi-mob encounters which will cross debuff every mob hitting the MT. On single encounters it may not be noticed.
Some top guilds out there swear by Mystics on high intensity encounters if they also have a dirge. Supposedly Mystics helping a Dirge proc stoneskin for the MT can make a big difference when tackling certain mobs. Works like a successful avoidance check by mitigating damage outright.
Helpful if your MT is taking slow but very big hits. Not so helpful on fast/medium hitting mob or mobs with light adds where stoneskin can wasted mitigating small hits.
Having a Defiler in the MT group is perfectionism. While the skills of the player matter most, fact is Defiler/Templar/Warden/Dirge/Coercer each have better buffs than their counterpart for the MT group. You can switch each class out and still be successful, but with all the "perfect group" preaching on the forums most MT's will choose these classes first. Only if the player fails horribly will they be replaced.