Interesting idea, but I highly dislike it. The entire point of email is that it is the "leave a message and I'll get back to you" mode of communication for the Internet. It is not the "respond right now" mode - that is what IM and chat rooms and other real-time communications are. I have things to do too, you know, and keeping track of someone else's schedule in order to be able to send them an email isn't on the lis…
You are free to decide if your gem of wisdom is important enough to actually notice if I get it, or just fling it into the ether. Your call.