Taking all the perks will allow you to be more effective with single casting a spell, ultimately saving you magicka, but as almost everyone who uses illusion grabs dual casting.

You don't really need both the middle and right path of perks if you do. Mages should just take the middle path, and characters that are more stealth should just go up the right, as mages don't really need quiet casting. With dual-casting, even the adept level frenzy spell will affect NPCs up to level 52 if you take just kindred mage, up to 57 if you have only taken Rage.

I just have noticed how everyone says that Illusion is a huge investment, but it is really no more than any other magic school.

I agree with this

The right path has silent casting, which is sort of useful.

I agree with this.

For me, In this game you don't really need to have anything "useful". Why did I pick every perk in the lockpick tree, even though it is the most useless tree in the game? 'Cause my character was a master at lockpicking and I felt it was necessary to "waste" perks on it. Even unbreakable picks and then master lockpick. I made a "pure mage" that capped at level 48 with all the mage perk trees maxed out (except conjuration and restoration 72) and have 2 or three perks left. Where would I spend those perks points if I didn't use them for "useless" perks? In 1H? He doesn't carry any weapons.

Yes. I was so glad to learn that Illusion Dual-Casting doubled the level of the spell. That made going through the skill tree so much more comfortable for me.

I've only spent into Illusion with my Kahjiit who specializes in Illusion and Restoration. My mage actually invested in Block instead because he has the magicka and ability to use all the Illusion spells effectively without any perks, and Block was more necessary to how I wanted to play him. Hes invested mostly in Alteration, Destruction, Restoration, and Block.

silent casting, duel casting, and a few "it-works-on-higher-levels-of-this" perks. Surpassing 100% reduction with enchantments make the game rediculously easy, but even a small amount of reduction can be managable.