Agreed. I bet most people leave email unread in their inbox so as to give themselves a sense that they are important, too important to read and/or respond to an email.
The concept of inbox zero isn't about "zero unread messages" - it's about "zero acted-upon messages in your inbox"
I make a point to mark everything as read pretty much the moment I get email, in any of my email accounts.
But my inbox is still littered with everything. My Gmail is a lost cause. I have things filtered through labels and whatnot, but... they are still in the inbox, all ~25,000 emails.
I keep telling myself to get with the inbox zero trend with my work email addresses - make folders, utilize them for things I've read that I don't need to worry about anymore (or, categorize things and mark unread again).
At this time, anything that really needs my attention in the near future but I can't get to immediately, I mark unread after reading, or throw the red flag on it in Outlook.
But I need to delete that which is unnecessary, and move to folders that which doesn't need any attention. My goal, if I can get around to it, is to only have emails in my inbox that still need attention, read or unread, flagged or not.