I'm planning on starting with a new character that will be mostly a thief but specialized in Conjuration and Illusion also. Now my question is? Is Illusion worth it?
Because I'm a bit scared off by the levelcap each spell has. Are there, say, at level 50 (or higher) still enemies that can be affected by the spells?
Something that I'd wanted to add is that I don't want to join the College of Winterhold so I don't have acces to the lvl 100 Spells.

If you are doing conjuration, yea. Illusion spells are really powerful and even just getting quiet casting, muffle and invisibility make your character a boss. The only spells that are affected by caps are fear/charm/frenzy all of which are OP. I don't know the technicalities of how the level caps work, but you're playing most of the game under level 50 anyway. It's not an MMO.

I joined the college just to get access to purchasing the invisibility spell. I reread your post and I'm not sure if you're asking whether or not to "master" illusion. If that's the case, then I would say that going to expert level so that you can get invisibility is as much as you really need as a thief.

If you dual cast the spells they are pretty strong. If you are a Vampire they are even stronger. Strong enough for the game to become ridiculously easy. Still, without being a Vampire dual casting spells like Pacify will affect anything to technically around 60 something.

EDIT :
Pacify 20
Hypnotic Gaze +8
Kindred Mage +10
That's 38 so Dual Casting will be level 76. Maybe a little higher because I think DC is +220%

So when I increase the power of Illusion spells does that translate to raising the levelcap of the spells? (i.e. do enchantments and dual casting increase the levelcap?)

The power of the spells ie levels, are set. Enchants only affect the Magicka cost.

Though potions of fortify school increase the power.

The level 100 spells are not worth it in my opinion as they are master spells and can not be duel cast. For example duel casting Pacify works on city guards when you have all the perks needed but Harmony does not. Other than that though, illusion is awsome. Especially when things are a bit dull and you want to liven up the town a bit. Nothing more fun than casting a few Frenzy spells and watching the ensuing chaos. And as a thief you definitely will love Invisibility and Silent Casting. Muffle won't be of much use after you get your guild gear but is cool for levelling.

ONE WORD INVISIBILITY!!!!!!!!!!!!!!!!!!!!!

I really only use the master spells to make people kill eachother in the cities...

If you don't join the College, you will have to get higher level tomes through random loot. For the top ones you will be waiting till after level 35.

Dual casting and perks raise the level caps. Someone said potions do. Other than that I don't think anything else affects it. Should still be good at high levels.

Illusion is easy to level up, buy Muffle and cast over and over again till you have had enough.

combine this with the college quest to clean the portals and bamn you can duel cast this bad boy for like 15-20 mins to get very high in illusion super fast.

imo illusion is the ultra master skill, my pasifist has 100 in illusion he can go anywhere do anything and not worry. lol calm ur way through the masses.

Nah, you don;t have to join the college as a mage to get access to the spells. During the MQ you have to enter the college and you can do so as "Dragonborn".