The hill fort allows you to upgrade those creatures whose dwelling upgrades have been built in controlled castles.
In this case, the fort would be a cheaper and, in some cases, more convenient upgrade option, which would justify its value as an object.
I'm not sure about Iskra and Vidomina; on the one hand, their flaws certainly won't allow them to become main heroes, but on the other hand, it's a really significant bonus to the number of bones in the Fortress of Bones, especially on Hard and Expert difficulties.
I never thought about Mullich; I think he could be unbanned, as he is unbanned in the Versus Community League, and you can't even start with him.
Regarding the level 4 specialists: yes, they don't really matter on Hard difficulty. But besides Hard, there are also Easy and Medium difficulty maps. On the same custom map, an early encounter in the second week with Solmyr with, say, 6 specialists and 2-3 Lightning spells can end very badly. On Medium difficulty maps, mages are much stronger than warriors in general, and level 4 specialists are especially so, even with the existing conditions for building a guild; they are very difficult to counter.