Just noticed there is a spell called Powerwell that temporarily gives you 5 to 20 spell points. This would be nice except that the spell costs 38 spell points to cast, making it useless.

What is the purpose of this spell other than to unlock the Fortify Magicka enchantment?

A Fortify Magicka spell would not be, however. All it would do (and I tested this) is allow you to overspend your Magicka; once it wears off, the game keeps track of your negative Magicka. (50 Magicka from a Restore Magicka left her with only 4 Magicka remaining in my test.)

Note that Fortify Magicka on self spells are only viable with short durations; otherwise the cost exceeds the gain (which is the problem with Powerwell).

I believe the combination of Fortify/Drain Intelligence and fortify Magicka could lead to an exploit which is even possible with constant effect items.

I have found another significant exploit that uses the Fortify Magicka effect.

Create the following spell (the order of effects matters here):
Drain Intelligence 1 second (to as close to 0 as possible)
Fortify Magicka 1 second (higher is better here)

When this spell wears off, you will have more Magicka than before. Even better, this ignores your normal limit, and it actually works best if you don't have a bonus from your race or sign. I now have over 4000 Magicka, which is over 20 times my normal limit, and the extra won't wear off until used or stolen.

Now to force the game to divide by zero and see what happens.

Edit: Division by zero. If you lower your Intelligence to zero and Fortify Magicka, the trick doesn't work. Your Magicka will by equal to its normal maximum minus the fortify magnitude.

Knowing the spell effect lets you use it in enchantments

Why put it in an enchantment when I can put it in a spell after a Drain Intelligence effect to get tons of spell points?

maybe I dislike abusing game mechanics?

Can you think of a truly worthwhile use of the Fortify Magicka effect that you would not consider "abusing the game mechanics"?

Also, getting endless Magicka sounds like the sort of thing that could be fun. I have already used it to keep my "Haste 100 points for 9 seconds" spell (cost 50) up while walking around; it's much more fun when I can go fast.

As an enchantment effect Fortify Magicka can be quite significant compared with any point in the "normal" range of magicka reserves... 100 Int Atronachs only get 200 Magicka. A pair of pants can be enchanted to provide +12 as a constant effect, much better as a triggerable effect unless you're building a whole magesuit.

Actually, I believe a 100 Int Atronach gets 300 Magicka, and a 100 Int Altmer Atronach gets 450 Magicka.

Also, Fortify Magicka has another downside: The Magicka provided counts against your maximum, so you need to remove the item before you rest or otherwise recover Magicka to regain it.

Honestly, if you want to get more Magicka with constane effect enchanting, a better choice is Fortify Intelligence. Your maximum Magicka will actually increase, allowing you to benefit from Magicka recovery effects. Also, if you are an Altmer or Breton, or have the Mage, Apprentice or Atronach birthsign, you will get more Magicka per point of enchantment.

Ah, yeah, forgot the possibility of racial magicka bonuses (I mostly play khajits & imperials), but normal, unbonused magicka at 100 int is 100. So Altmer Atronachs get 450, I think you're right. Still, a full magesuit (maxed out constant fortify magicka enchantments) on exquisite pants, shirt, belt, shoes, skirt, robe, 2 rings, amulet, and left & right extravagant gloves (best that exist) will total 140 magicka, which is nearly the full Altmer bonus at 100 Int. Not too shabby, but a lot to build up to. An exquisite ring that fortifies magicka when triggered backed up by a Golden Saint (or ... Amalexia) soul would do quite nicely as an alternative.

Thing is, for the same amount of effort I could get constant 140 intelligence, which would be better for several reasons:
1. It is not necessary to unequip the items to regain the extra Magicka (though it helps because of the exploitable mechanic that current Magicka scales with its maximum).
2. For an Altmer Atronach, I would get 630 Magicka with the intelligence setup. (My Breton Apprentice would get 420). Low Magicka setups still get 140, so you aren't doing any worse.
3. Intelligence has other benefits. With more Intelligence, I can make better potions and my enchanting are more reliable.

True enough, fortify int does seem like the better game mechanical choice. Really, fortify attribute ought to be more expensive than fortify magicka since I can't think of a single case when fortifying magicka would be preferable... it's really just a trap option. Is fortify magicka significantly more easily obtained in a spell than fortify attribute? I imagine the opposite is true.

There are plenty of Fortify Attribute spells available for purchase. On the other hand, the only Fortify Magicka spell available is Powerwell, which as I mentioned, is overpriced.

Maybe Fortify Magicka should have cost about half as much, say a base power of .5 instead of 1.

Also, Absorb Health should have been a bit more expensive, maybe in the 10-12 range. (More than Damage Health, but less than Damage Health + Restore Health.)

It looks like Water Speed is also overpriced.

Maybe there needs to be a mod to rebalance spell effect costs. Is there one?

(Daggerfall also has issues of this sort, but some are more serious, like the fact that creating Regenerate Health 30/level per round is far cheaper than it ought to be.)

Another observation: A similar trick works for Fatigue as well. Drain the relevant stats to zero (except 1 that should be left at at least 1), then Fortify Fatigue and your Fatigue is now well above its maximum. This allows you to, for example, reliably cast spells that are otherwise too difficult, as well as everything else Fatigue affects.

Health is now the only thing I can't get high with exploits, but UESP wiki mentions that apparently jumping with ridiculous fatigue may give you more when you land. Maybe I should test this.