Why? because it means I don't feel the stress of having to zero something out. Sure, the more unread stuff I see, the more I know I'm backlogged...but I find it handy to see recent emails toward the top... and that's what leaving everything in the inbox does for me.
I admit when things get crazy at work, Priority Inbox works well for my system.
Overall I doubt any good comes from these loose email practices. I may have acquired business style email use earlier in life from the freelancing so I may not be considering their situation.
When an email comes in I never leave it over a day to respond. Just two days ago I got into a real-time email conversation with someone trying to compile my open source project. I invited him unto gtalk and solved the compilation issue in <5 minutes. Had I taken longer to respond he would have gotten bored long before he could become a potential contributor.
I know people that archive all mails and do respond slow in general and people that respond fast.
I also never saw what archiving an email should achieve. Having it unread is archived enough.
My work account however is organised into folders, I normally just leave items as unread until I've replied or am finished with them, then they're archived into folders.