I just copied that rule from the other game, but personally I don't see any real issue with Diplomats using PMs. It seems to me a big part of this game is double dealing, trickery and misdirection, and that's more easily done privately. If you RP it, Diplomats wouldn't always be talking in a group with everyone, when they know Assassins are among them. They'd be talking in small groups as well, to the Diplomats they're reasonably sure aren't Assassins. And without PMs, the usefulness of the Sage's ability is significantly lessened.
In the interests of experimentation, I'll remove the "PMs discouraged" rule entirely for this game, so people can PM as they wish. Then next game I'll ban non-Assassin PMs, and we'll see which one makes for a more fun game.
Edit: I'll also point out that PMs were not used "extensively" in the other game. The whole "convince them I'm innocent and get them to vote for a villager" thing involved all of three PMs from me.
