At one time, I considered that Boros Aggro could play a singleton of the Diplomats to force opponents to run into their Boros Reckoners or first-strike Ash Zealots, but that dream never came true. That being said, he wasn’t a bad card in Magic 2014 Limited (which was a brutal format, in my experience). He just is a bit weak for Constructed play, as he’s not as aggressive as you want a Goblin to be.
However, in Commander, it’s a different story. Causing chaos on the board is some thing red players love doing in Commander. Forcing an opponent to make their creatures attack is pretty mean. It can cause a lot of drama in a multiplayer game. Goblin Tribal decks often can find room for this guy, and he was an auto-include in my Krenko, Mob Boss deck. That is the Commander deck in which the Diplomats see the most play.
He also happened to be the rare Top 8 foil promo for Magic 2014 Game Day… oh, how exciting!