Master is useless, as are the fear and rage specific perks although some disagree there. Personally, I get by far the most use out of pacify, have never used fear, and frenzy is mostly unnecessary although it has a certain novelty.
Dual casting is very important, quiet casting is one of the best spell casting perks available, and everything that improves the level your calm, and eventually pacify, spell can effect should be taken as soon as possible.
I've found frenzy helpful on many occasions, especially since many boss type characters are surrounded by their weaker minions, it's a good way to weaken them. i fnd it's more helpful than pacify in this particular situation. It can also help clear out a fort, especially with enemies you can't see behind the wall... just use it on the guys on the towers and they'll start shooting the people inside.
I do agree with the things you mentioned about dual casting and quiet casting.
Pacify IS the most useful out of all the perks, Frenzy is moderately useful, and Fear I largely ignore.
As far as which perks to invest in, Illusion is the tree that benefits the most from investing in everything. If you want to save a few perks, don't invest in the middle branch (Hypnotic Glaze, Aspect of Terror, and Rage). I'm at level 41, and I still find I can use illusion on the majority of enemies in the game. If they're powerful, I just dual cast.