No surprise. Illusion has no beneficial perks like the other trees (regeneration and +% mana regen, magic resistance, soulbinding weapons, atronach) only silent casting which does who knows what. The main spells there are frenzy, calm and fear. All utterly useless. Playing master and Ive yet to see a situation where these would be better than pretty much ANYTHING else.
You need to be creative. Calm is incredibly powerful once you realize it should actually be called "free sneak attacks". Combine that with invisibility (or muffle at a lower level) and you can clear a dungeon without getting hit once. Frenzy is the "make opponents stop chasing me" spell so if [censored] goes south, you pop a frenzy, retreat and stealth back in.
It's actually overpowered, as there isn't anything I can't sneak attack anymore.
Edit: and yeah silent casting means you can stand right behind somebody while casting buff spells.