Here's a post from the recesses of memory.
Back in 2004, Brent Simmons said:
… your feedreader, like your browser and email app, is a hub of information. It makes sense to want to route information from the hub to other applications.
This is a great point - and I'd like to expand on it. I think it goes further than just being able to send data from your email client to other programs. Data should be easier to send in both directions.
Most email clients want to be your only interface to your mail data. Since email communication can affect everything we do on a workstation, every program should be able to look at and update your email store, even triggering checks or controlling how you are notified of new messages. What program is better able to determine what you're likely focused on than the one you're currently using?
The best way to handle this would involve some system-level support, so the largest set of applications could use it. I'm not holding my breath…