It's open in name but proprietary. How many competing open source clients exist? Right.

Also, it's extremely limited like every other proprietary system, by the fact that the client is proprietary and thus can only do what they feel like supporting.

How do I pipe incoming signal messages to a script which will process it in ways I like? How do I programmatically send messages based on the output of other scripts? And so on.

All this is trivially easy functionality with email because it is an open protocol.