How important is it?

I'm lvl 39 atm with main stats
sneak/pickpocket/alchemy/lockpick/one-handed (no archery because it makes it easy mode imo)

I haven't spent anything in magicka/illusion and the only time I
press the spell button is for the shadowcloak of nocturnal/throw voice shout

I'm just curious on how much it helps a thief/assassin build because I have the
nightingale armor and the hood for 17% less illusion magicka.

Halfway through the Thieves Guild, I got tired of chugging invisibility potions and headed to the College to train my illusion. I got it up to 75, learned invisibility, and quickly found out that casting Invisibility alerts everyone nearby to your presence. There is a perk on the Illusion tree that makes for silent spellcasting, but I didn't want to invest in the Illusion tree for just one or two spells.

So. In my opinion, the most worthwhile spell is Invisibility, but it has to be cast well of out sight of NPCs or they'll see you cast it. If you have advanced alchemy, I think potions are better, albeit shorter lasting.

Yeah my alchemys actually pretty high already.
Maybe 89 with 6 perks invested (Not getting smithing/enchanting)

I'm just wondering about some of the illusion skills like pacify or mayhem.
I guess I could always just paralysis poison by weapon or pickpocket
Even though that's technically alteration lol

+ I won't spoil it for you, but you can get an invisibility like spell from the Thieves Guild

I found alchemy, smithing and enchanting more useful.

Illusion is too powerful for me, one of my character specialize in illusion and he just walks right up to a dungeon cast mayhem on a group of enemies then they kill each other by the end of it only one stands alive and I shoot it with a bow and he dies, that's it.

with a high enough sneak skill, for all intents and purposes you are invisible. once i hit 70 sneak and got the perk where movement wasn't added into detection it was over. then i took shadow warrior and it was really over. if you're going to use illusion for invisibility or muffle, don't bother. sneak is more than enough. you also only need to take the first level of the +20% harder to detect. everything beyond that is overkill.