Seth Godin, in an open letter to Google:
Imagine that your mailman takes all the magazines you subscribe to, mixes them in with the junk mail you never asked for, and dumps all of it in a second mailbox, one that you don’t see on your way into the house every day. And when you subscribe to new magazines, they instantly get mixed in as well.
I briefly turned on Google’s separate Inboxes feature a few weeks ago. I found it maddening and turned it off within a few days.