Is diplomacy necessary?

I don't know. But I've repeatedly noticed that at a lower difficulty level, monsters are more likely to join on their own than at higher levels.

Well, you're wrong about that... Maybe you're talking about player-created maps? But in random maps generated by the editor, the computer ALWAYS sets the 4th joining level, i.e., "unlikely to join."
I'll also correct Amber a bit about what he said, that if you play with the forces of your own castle, there's no point in using diplomacy... it depends: for example, if I play as Conflux - 3 monsters with immunity to fire, and with diplomacy, I can easily find 4 more, and then Armageddon - it will be quite difficult to deal with that ;).

I'm very happy for 90% of the players, but the computer doesn't get any smarter above "Bishop" difficulty. Then it just starts reducing the player's resources and increasing the computer's resources. In my opinion, it's a pointless challenge. But this is already off-topic.

Here again, you're mistaken - even directly in the battle itself, the computer changes its combat tactics, for example, it forgets to "wait," which gives an even more convincing victory over it :)