Your favorite hero...

I agree that it depends on many factors. I just listed those that I personally like more in terms of specialization, but I don't always play as them.
Deleb was nerfed in the latest expansion, so I don't pick her very often. If you need speed, Grok is a good choice.
Letos is useless against Necromancers. But at higher levels, Witches give a good damage boost to Erinn.
Ora is always a good choice for Mages, because the hero's spells play a bigger role than their army. Many of the units there aren't very good, except perhaps with artifacts. Otherwise, I agree.

Well, it goes without saying that there's no point in taking Letos against Necromancers. As for the feasibility of choosing Erinn:
1) At the beginning of the game, there are no Witches in the army, or there are very few. They mainly serve to support with spells. They are unlikely to be called great archers, and it is unlikely that anyone will enhance them by taking the Archery perk, for example.
2) In a long game, the effectiveness of Destruction magic decreases sharply. In order for the Witches to fire, the hero must cast some MP spell. Well, the hero will destroy 10-20% of the enemy stack (maybe even less) with a spell, and the Witches will kill about the same amount with their shot (if they fire and if they haven't been killed yet), and will that really help? It is much more effective to cast Berserk / Mass Haste, Stone Skin, Dodge / summon a Phantom or Phoenix, IMHO.
Against the same Necromancers, I would take Iruna. She has a specialization in main units, the ability to develop any school of magic, and Education as a basic skill, which will solve the problems with mana and allow you to quickly get a mentor. And this, in turn, will allow you to constantly send upgraded Beetles with a few Stalkers to the enemy, which will significantly reduce the Necromancer's troops.