Is diplomacy necessary?

If you're playing as Necropolis, it makes sense to take the initial monsters and level them up using the Skeleton Converter spell. In general, Diplomacy is only useful at the beginning. Later on, in large battles, it becomes meaningless. Overall, I consider Diplomacy to be a useless skill for me, and the aforementioned reasons are just minor advantages.

In what large battles is this the case? How many months are you planning to play? 10? For example: a hero descends into a dungeon, recruits, say, 120 Demons, 30 Naga Queens, 59 Mighty Gorgons, 25 Titans, 45 Catapults, 200 Magogs, and 17 Angels... By the end of the first month, all of this comes to you.
Question: how (or what) are you going to use to keep all of this without Diplomacy?