I think a 30, 40, 50, 60 hour employee can be very different things. A 50 hour employee who isn't productive is clearly an issue. A 60 hour employee who is clearly burned out BUT productive is a problem too.
I don't think that hours are the first or only thing by which an employee can be judged. It's more about setting a standard and allowing the manager to monitor and enforce that based on the individual.
My experience might be a bit jaded. I work for a company that is "small", very close (many 15+ year employees). So, time spent "in the office" doesn't necessarily mean we're locked to our chairs and keyboards.
Of course a 50-hour employee is productive, but would you say consistently over a longer number of weeks he's 12.5% more productive than a 40-hour employee?
To give myself as an example: I work out 3-4 times a week, spend my time effectively reading books, playing the piano regularly and spend some quality time with my wife. These activities I would not be able to do would I work consistently 60+ hours a week. Nonetheless, it has had and continues to have a profound positive impact on my energy levels, motivation, communication and creative reasoning.