It's useful to know what features the other client has... no point sending dm-s to someone whose client doesn't implement dm-s at all..

Coracle already adds a 'client' field somewhere by default I think. Gossip has an option for it too...

Then again, 1 post coming for a cetrain client doesn't (and shouldn't) imply that the other user is using that client consistenly. Still, it's a simple enough feature..