Back to original thread intention, my personal stand is if you are a graceful citizen and knows to give up your seat to those in need more than you, you don't have to be in any reserved seats to exercise such a behavior.
In my opinion, the reserved seat is only for those that doesn't understand what it means on whole. While it is obvious why the reserved seats are near any entrance, I believe the notion extends further than that. Some people need a sign to tell you what to do, then well you know what it tells you about yourself. Some people can have self initiative, that is a good thing.
From the perspective of a government, they can only encourage you to do so via notice placed, but do you really need someone to tell you that or you just need someone to faint or fall right in front of you then you will understand that why giving up your seats at appropriate times is important.
As such, I don't think reserved seats should be left empty. It is silly not to fully utilize the resources. Why is there an empty seats when the cabin is full of people standing? Doesn't make sense. Hence it only make sense to give up your seat whether it is reserved or not, to those in more need than you.