Playing as the Legions (Disciples 2).

In short, I didn't understand the developers' intention behind introducing this unit into the game.

It's more fun with it!
The transformation is really pointless, IMHO, but there should be some interesting feature. Although I don't really like them either; I tried it a couple of times and then stopped playing.

Neutrals definitely have some unhealthy interest in it.

By the way, I try to take advantage of this because I'm trying to level up two possessed creatures equally. That's why I put the gargoyle in the center. If they kill the gargoyle, I'll explode, resurrect, but if they kill one of the possessed creatures, I'll have to wait for both to gain the same amount of experience.
In general, the unhealthy interest is explained very simply - it's an archer, on the front line, how can they not attack it???

I never keep two two-tile creatures in a demon army.

And that's right. It's boring. A well-leveled Hell Warrior hits almost as hard as the Demon Lord, and it frees up space for a mage.

You can't see anything.

My army is practically transparent. Gargoyle in the center (the onyx one is compact), Hell Warriors in front, Arch Devil and Incubus in the back. Sometimes I switch the Arch Devil and Incubus, because one of them is still hard to see.
I often have the urge to freeze the warriors at the Champion level, but I quickly dismiss it - Hell Warriors regenerate faster, and they don't look much different anyway.