Is diplomacy necessary?

Silly question: what difficulties could there be with it?

The main difficulty is that it takes up a slot among the secondary skills! :) It's better to give Diplomacy to a secondary hero who will also be a squire (in the sense of carrying around various useful and not-so-useful monsters) and who will pass them on to the main hero before the battle. And for the main hero, Diplomacy is usually an unaffordable luxury... Only Eagle Eye and Mysticism are probably worse... :)