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 list. The whole point of email is that I can send on my schedule, and you can read it on yours. Now you're taking that away from me and forcing me onto your schedule.
All one needs to do is put a vacation autoresponder on their email account, and then have the self-control to not read email while they're out.