If I need something, I just search for it. As the Gmail search is really, really, good, I can pretty much instantly bring up any thread. I therefore don't see any value in spending even a second of time in trying to organize a piece of email. I also really like the "Social" and "Promotions" tabs Gmail added, it's like a smart filter for "unimportant stuff" that I don't even need to bother looking at but can search for if needed. I look forward to Inbox automating and highlighting actionable items even more (check in for flight etc, should just be a button press without even opening the email).
I would rather apply a label, or mark a message as unread (this is what I typically do), if it is something I need to return to later. Most emails can either be directly acted upon, or don't require a followup at a later time, so optimizing the common case down to "do nothing" makes sense for me.