In Daggerfall and Morrowind, the amount of skill experience you get for casting a spell does not depend on the spell's cost or power. As a result, from a practice perspective, it is rather pointless to make expensive spells. In fact, this encourages the repeated use of weak spells instead of using stronger spells.

Another issue is that, at higher skill levels, it takes more work to improve the skill. That would be fine if you could get more skill experience by using more powerful spells, but you can't.

This issue doesn't affect Arena because the game doesn't use the skill system. I believe it does affect Oblivion, and I'm not sure about Skyrim, but then again, I haven't played either game.

Of note, Dungeon Master gives you more skill experience for using more powerful spells, and I believe Wizardry 8 does so as well. (The latter actually scales it to spell cost, and both games give skill experience for failed casts, which is nice.)

Dungeon Master actually had a surprisingly deep system under the hood, opaque to players.

it's not a flaw
it's just the way the game is
also there are many examples in real life where a person can train a skill by using simple inexpensive means

ps you should definitely play the newer games too