Merlin Mann is doing a series on keeping your inbox level down. I like what he has to say about leaving a message in your inbox without acting on it / filing it / deleting it:
Just remember that every email you read, re-read, and re-re-re-re-re-read as it sits in that big dumb pile is actually incurring mental debt on your behalf. The interest you pay on email you’re reluctant to deal with is compounded every day and, in all likelihood, it’s what’s led you to feeling like such a useless slacker today. Maybe? Think about it.
Email programs have not changed much in the past decade, but the amount of email we get has grown by a tremendous amount, significantly impacting our email productivity. The average information worker gets far more mail than s/he can cope with, and an increasing number of people suffer from "email overload". In this blog, Itzy Sabo analyzes the causes of email overload, discusses strategies to cope with the constant bombardment, and provides practical tips for getting the most out of our email programs. 
