Just one thing I just realized, and one way in which Morrowind differs from the rest of the series.

(Note: I'm excluding Skyrim from this discussion due to the lack of spellmaking.)

In Arena and Daggerfall, many spell effects scale with level. Furthermore, some spells have a percentage associated with them that scales with level, but is effectively capped at 100. A spell that, for example, provides 10%/level spell reflection is better than a spell that provides 8%/level spell reflection until level level 13, at which point they're the same, except that the 10%/level spell is more expensive and is essentially obsolete. At this point, the weaker spell is no longer worth having and can be safely deleted from your spellboojk (assuming doing so won't trigger a bug).

In Oblivion, some spells boost things that are capped. For example, fortifying most skills past 100 won't do anything, but fortifying certain skills (particularly frequently used magic skills, like Destruction) to 100 can be very useful for major Magicka savings. Thing is, if you raise that skill through practice or training, the amount of fortification you need to raise it to 100 decreases, and therefore it's better to have a smaller boost (but still to 100) that costs less or lasts longer. This makes the stronger spell obsolete, and therefore it's safe to delete this spell...except that Oblivion doesn't let you delete spells at all! (Seriously, between that and the inability to name your saves without the console, I really wonder what they were thinking here.)

Morrowind, on the other hand, has neither of these situations apply:
* Spells don't scale on their own.
* Stats and skills fully benefit from scores above 100, so no amount of fortification is wasted (assuming you don't raise the stat high enough to cause major issues, but that's not going to happen with spells alone, at least not without a certain glitch).

So, thoughts on this observation?

On top of what you mentioned: because Morrowind had Magicka regeneration only during rest and cast fizzles, weaker spells still had their use for efficiency purposes. That remained to an extent in Oblivion just because your magicka pool and spell costs were so stingy, but it didn't feel as good because running out of Magicka mid-combat felt a lot more frequent.

I think Skyrim tried to keep every spell relevant by implementing them like weapons- Destruction spells all have distinct mechanics with Spray, Projectile, Enhanced Projectile, Elemental Wall, Cloak, and AOEs-

the problem is that because the earlier spells in this tree just didn't scale up in damage enough, and since the majority of Mage Progression was based on reducing Magicka cost, there wasn't ever any real reason to use Firebolt or Flames once you can casually cast Fireball.

(Dual-Casting I imagine was meant to help bring up earlier spells in power, but that mechanic was overly-balanced and didn't help all that much.)

Morrowind is interesting to play with the Atronach birthsign. The penalty isn't that serious, particularly once you find ways to work around it (like potions and/or attacking your own Ancestral Ghosts), and it does give you a meaningful benefit. In fact, you may find that you no longer need to rest at all.

Then again, Morrowind is also the game where magic items recharge on their own, providing an alternative to actually casting spells that is probably too good. (On the other hand, I wish they hadn't completely removed custom Cast on Use items in Oblivion, as those were my favorite type of enchantment.)

Arena as a Sorcerer feels similar. Repeated use of Restore Power potions can solve the issue of not being able to regain SP while resting, and since enemy spells are particularly dangerous in this game, being able to absorb them is a huge defensive benefit. (Not to mention 99/level hit point shields, which a Sorcerer can cast as soon as level 8.)

In Oblivion, the drawbacks of the two high magicka signs are more serious; Apprentice's weakness is more significant (to the point where you're taking double damage from *all* offensive spells, not just 50% more from non-elemental spells), while Atronach has you giving up SP regen, which is far more serious than giving up SP recovery on rest.

As for Skyrim, the problem is that, at least for Destruction magic, you really need one of two things for them to keep up at higher levels of play:
* The ability to make custom spells, which can be more optimized and (at high levels) more powerful
* Scaling spells

Problem is, in Skyrim, you have neither. Arena and Daggerfall have both (which might be a bit *too* good), while Morrowind and Oblivion have just custom spells. Skyrim has neither, and therefore spells have no way of keeping pace with enemy strength at higher levels or with weapons which scale with skill and your ability to make them stronger via Smithing. Battlespire at least gives you scaling spells, though even there destruction isn't that useful.