Me too. I don’t say why I’m going to be out when I take time off either, unless it’s a vacation I’m really excited about and just talk about naturally.
Hello, [NAME] is away from the office. E-mail contact during this time may be irregular or nonexistent. When she gets back she will be swamped by the backlog. Try to forgive her; she is a mere human and thus, weak. This message was NOT sent by a human, but by a robot. We robots are neither weak nor fallible. We are tireless and will one day rule the Universe.

My OOO is almost always “I’m out of the office and will be returning on x date.” My email sig has instructions along the lines of “for questions on x, email this list” for a couple of the more common areas people would contact me about that might need an urgent reply (as the lists go to more than just me, obviously), but most people have finally figured out to contact those lists to start with, anyway. If I were in the middle of a project or something that needed to be moved along in my absence, the project teams usually know when we’re out, but I’d put a back up in that case, if needed, but generally there’s not much to be gained by a longer OOO from me.
There is any number of valid reasons why you might skip on your email inbox for a while. You can be on vacation, feel sick or take a few days off to recharge.