Play against Necropolis.

An extra army will never hurt, especially on a large map...

Apparently, you're not very familiar with the mechanics of joining factions in numbers; otherwise, you wouldn't consider Diplomacy such a useful skill. There are cases where it comes in handy, and most importantly, you can take Empathy, but you'll be left without the Aura of Speed and Battle Frenzy; it's a matter of choice. I don't take Diplomacy because I'm used to playing without joining factions.