I wouldn't say completely useless, but it's much more convenient to use a potion during combat than having to unequip your weapons/spells to heal, then re-equip them.

But using a potion is only going to heal you. Id rather level restoration and take advantage of the other benefits.

You forget that you don't necessarily have to put points in a skill even if you use it a lot. Lockpicking is a good example of this: you'll naturally level it quite high, no matter what kind of build you have, but it's not really worth putting points in. So you can use all the skills you mentioned, but you don't necessarily have to buy every perk in every tree.

If you're looking to streamline, though, my advice is to decide whether you want a sneak thief/assassin or a sneaky mage, and then pare down your skill lists accordingly:

Sneak Thief/Assassin: Light Armor or Alteration, One handed (daggers), Archery, Sneak (put points in), Illusion, Alchemy, Pickpocket (just up to Poisoned or maybe Extra Pockets). You'll probably get quite high levels in Lockpick and Speech, but you don't necessarily need to put points there. With this build, you can get lots of sneak kills, either from afar by poisoning your arrows, or up close by slipping poisons into unsuspecting people's pockets. Alchemy is better in this instance than Restoration because to get one-shot kills using poisons, you'll need to make them as the ones you find won't be strong enough. You can also customize them: Slow/Damage Stamina poisons for melee enemies, Damage Magicka/Damage Magicka Regen poisons for mages, and Paralyze poisons for everybody! Your one disadvantage until you get Illusion to level 90 will be that you cannot use spells (or poisons) on undead or automatons, nor are dragons ever vulnerable to Illusion spells (no matter what your level), so you will have to rely on weapon damage to take them out (although dragons are susceptible to poisons). The perks upping sneak attack damage for both bow and dagger will be invaluable to you. You won't necessarily need to level Conjuration, so that may be a place to save some perk points, as well. If you find yourself in need of back-up at times, you can always get the Sanguine's Rose staff: a Dremora Lord is a pretty good Oh Sh-! button!

Sneaky Mage: Alteration, Illusion, Conjuration, Restoration, maybe some Destruction and Enchanting thrown in. You'll level Sneak quite a lot, but you won't necessarily need to put any points there unless you want to. This one is the complete package: there's something for all situations here. Undead? Restoration spells, and Illusion after you level it enough (nothing like pitting Draugr Deathlords against each other). Getting swarmed? A Destruction cloak spell or two, then Frenzy everyone you can and conjure up some help. Facing mages? Wards and Destruction for counterattacking. Alteration spells to beef up your armor, and for Paralyze, which is godly and pretty much breaks the game. Enchanting would help to reduce the casting costs of your preferred schools of magic. I really had a blast playing a sneaky Illusion mage: sneaking around dungeons and bandit lairs pitting my enemies against each other (and then conjuring up an atronach to finish off any survivors) never got old!

I like how you've defined your sneaky mage. My one issue (meaning the part I'm not sure how to execute on in my own game) is the "pitting my enemies against each other". At level 16 (Illusion in the 40s somewhere, as perked as possible), my only "fury" type spell doesn't seem to work on anyone. Is this just a strat that won't work until I skill-up Illusion some more or is there a spell or approach that I'm missing?

thanks!

Nice, I was wondering the same thing. I was about to look up a chart that shows how strong the Illusion spells are and on what levels they work against enemies.

Is there a hotkey for health pots that I don't know about? On my (slowish) PC, I find going into the items menu to be much more laborious and immersion-breaking than punching my Heal-spell shortcut to put the spell in my left hand and then casting.

Yes there is! Once I realized it I was very happy. Just add it to your favorites like any other item!

Dual cast your illusion spell, it effectively DOUBLES the level that it works on.

With no perks, I think it should work up to level 30. UESP page has a little table that explains it...

Theres a perk in the Illusion tree (I forget the name of it) that allows you to cast Illusion spells on higher level humans. Theres also one for animals and eventually undead. That, along with higher level Illusion spells is why Illusion needs to be all or nothing for it to be most effective.

That could work as well.

See, that's what I hate about the tooltips in-game. I think the perk desc just says "Dual cast for an even more powerful effect". How the hell are we supposed to know what that means?

Anyhow, thanks for the tip, SerStiefel. I'll give it a try. Now if I could only be more accurate with my Calm/Fury casting! haha i'm always missing those bastards in combat. Hate that aspect of casting -- aiming should be only for you physical damage neanderthals!

I could be wrong but i think if you duel cast illusion there might be a wider hit radius.

What everyone's already said: Dual-cast it.

http://www.uesp.net/wiki/Skyrim:Fury

http://www.uesp.net/wiki/Skyrim:Frenzy_(spell)

Other spells http://www.uesp.net/wiki/Skyrim:Spells.

Now you're talkin' my language. Does it skill up Illusion more than single-hand casting too? Oh please oh please...

Actually, if your Illusion is already past level 40, you should get the Frenzy spell: it's an area of effect spell, so you won't have to be as accurate in casting it. It should be available to buy from Drevis at the College of Winterhold.

And if you're concerned about leveling Illusion quickly, get the Muffle spell and cast it constantly.

Fantastic, thank you!

And yes, Muffle increased my illusion skill every casting for the first 5 or 6 casts haha.

Yea even with my characters that don't use Illusion I'll do this just so I can increase my overall level. Very handy in the later levels when it's harder to level up.

Especially if you have the Mage Stone activated.

I also found that you can re-cast Calm on an enemy, even while he's still Calmed, and still get Illusion skill-ups.