What does that have to do with anything? We've got wizards and dragons, realism is not on the table.
Arthur was a terrible King without Merlin, and Merlin wasn't running around in the front lines, either. He was constantly busy, but not fighting.
The fun is greatly diminished when I'm leading troops doing everything, until the time comes to invade someone else. Then I don't, due to the game over mechanic.
You're still leading troops doing the same things you were doing before, now you have Generals rampaging through enemy territory. YOUR purpose hasn't changed.
It's particuarly bad as a caster sovereign, because that's your most powerful caster and you really shouldn't use them in offensive sieges. With how rare magic is on other units, it takes the "war of magic" out of the game entirely and it becomes "war of maces".
I don't see any situation where telling a gamer not to use their best unit is a good idea or in any way fun. It's terrible.
Eisenhower wasn't his armies strongest wizard, either. Running around to inns finding missing flutes is a pretty lame thing to be doing in the middle of a war.
Think outside the box. Your unit is powerful not just because he can cast spells, he can also give the ability to cast spells to other units, giving you a large number of casters of moderate strength. Having 4 moderately powerful casters (hell, even 2) is having considerably more power than having 1 very powerful caster, because they can be in more places at once, and they can cast more times in one turn.
If you spread around his power to a few primary generals, this then makes the Magic Tree infinitely more useful, because it has a number of "Unit Essence Bonus" techs and buildings that add multipliers (that stack like everything else) to every single unit you have that has essence, turning all of those moderately powerful casters into an army of death-wizards.
...and all of this comes about because your Sovereign, instead of being an arrogant, power-hungry fiend, placed his faith in his Generals, and imbued them with his own strength (from a storyline aspect
), which is a major task when you think about it.
Compound that with when your kids grow up and pass essence around to even more Generals, and then contrast that with an enemy who has one really powerful caster with a big army and is up against a bunch of strong armies with really powerful casters. You'd get rocked hard.
This of course goes further into making sure the AI uses these abilities as well, which is a current issue. Without MP, it makes it seem less attractive because the AI is not using it to make it appear more attractive.