The difference in skeletons raised at different levels of sorcery. Essentially, when this rule was introduced, necromancy was completely forbidden for these 2 heroes to prevent cheating, which is extremely difficult to verify. With Expert, she will raise more skeletons.
Imagine a situation where, for example, it's week 4, you have Galthran as your main hero, and Isra and Vidomina are also in the chain, and maybe a couple more necromancers, and the game itself ends around day 222. During some confusing and complex turns, you could cheat by not deleting skeletons or deleting fewer. Imagine you have external dwellings of peasants, converters, a bunch of different castles and cannon fodder for skeletons, lots of battles on the go, etc. Tracking necromancy would be extremely difficult, and you'd essentially have to track every turn when Isra and Vidomina appear for someone.
You're already bound by the rules: you can't do 2DD, use scrolls without a Mage Guild, the list goes on endlessly.
I was only writing about the fact that attacking the opponent and his objects with Isra/Vidomina is forbidden by the rules. In my opinion, there is no difference whether these heroes are level 30 or level 20, with Expert sorcery or without, when there is no question of raising bones using the aforementioned heroes. The point is that when you free a prisoner from a prison, you just need the hero himself, not his specialization or leveling. There are simply very many such situations, especially on Jebus Cross.