I know that Kobo monitors these forums but I think it's important that we let them know how we feel by sending them an e-mail to their help desk. If they base their firmware changes by the feedback that they receive there, as they say they do, then that's the place to go...
http://kobo.zendesk.com/anonymous_requests/new
Here's what I said
"Please please diasble the automatic power off in sleep mode. I don't want my kobo reader to power off. I want it to sleep. If I wanted it to power off I'd turn it off."