Basically, we need to relate working effectively with working efficiently. Another way to look at it is: is the person that spends 10 hours a day sitting at their desk any more dedicated than the person who spends 6? The person who spends 6 hours a day at their desk can potentially produce more results if they focus on effectiveness and efficiency (Parkinson's Law in effect), especially if the "dedicated" 10-hour-a-day person spends 5 hours checking email.

It's also a generational issue that everyone goes through at some point. The young people think the old people are "dinosaurs", and the old people think the young people don't have any work ethic. The focus should be on results, not hours spent.