I wasn't talking about a mindless rush on Pitfiends. I said that in a battle against a human player, even if I attack, I won't finish them off completely, and I'll also weaken the Pitfiends so they won't be resurrected too often. When playing against neutral creatures, you can use your units as bait, and they won't even attack the Pitfiends.

Maybe I'm wrong, but when playing against Inferno, I "always" attack the Pitfiends on the first turn, against Tower - the Naga, against Dungeon - the Minotaurs, against Rampart - the Elves. As a rule, Pitfiends don't last long.

These are units with a very high damage/HP ratio. That is, it is the fastest way to reduce the opponent's offensive power. In the case of Pitfiends, it is also a guarantee that an unexpected stack of demons will not suddenly appear at the most inappropriate moment.

Recently, I was playing, and my opponent kept falling for the same trick - my dogs blocked his archers for half the battle, and when he killed them, demons appeared in their place :)
An unpleasant surprise, to say the least.

Added after 25 minutes

I'll add about XP and damage.
You have 7 stacks. Let's choose the leftmost stack (harpies, spearmen, etc.) that cannot be included in these 7 stacks. And they will become demons, which will definitely have more damage than the units you were saving them for. And the demons will have better stats.

If we are talking about a "non-native" army, then I agree. But if we are talking about a native army, simple calculations will show that HP and Damage = const. Despite the higher stats of the demons.

There will be an effect of reducing damage variance. After all, demons have a fairly stable attack: 7-9.

Simple calculations will also show that raising demons will be more expensive than buying them. For example, 1 demon can be raised from 9 Imps.
9 Imps cost 450 gold, and one demon costs 250.

Therefore, let the Imps and other creatures solve some combat task before they die, instead of just being fed to the enemy :)