I think that "save anywhere" would not be very practical in Dom3. Just consider how save mechanism from more conventional strategy game worked in Dominions: Niefel Jarl attacks your militiamen and misses - save - both your militiamen attack Niefel Jarl and miss - reload - one militiaman misses, another hit for 12 of damage - ok, save - Niefel Jarl attacks and kills one of militiamen - reload - etc

Considering number of battles and number of combatants this doesn't look like something practical to do. I believe that the current way of "save at the start of the planning stage and anywhere during the planning stage" is the right choice. With simple script one can also add auto save at the end of the planning stage. You can't make multiple saves in the middle of planning stage, but it's not very limiting, because unlike other games you can change your orders, so it's not like you can't get back to some previous point.