Out of Office (OOF) is a common habit. We put it when we are not on our desk. But in today’s world of mobility, internet availability and smart phones, it should be used ONLY when you are really on LEAVE.
I have seen people pout OOF message when they are traveling on official visit, in off-site, undertaking training programs and so on. That is NO LONGER a valid reason for putting OOF message.
I do agree that your response will be delayed in such circumstances. But there are two related arguments in this context.
Firstly, people do not expect lightning fast responses in general. If you respond in few hours, it is ok. And whatever official work you have, meetings, flights, etc. you will get a break. And what do you do in the break? Check mails. So DO NOT bother to set OOF.
Secondly, even if you are in some so called engaging activity, most people are checking emails during off-sites, training programs, and even during customer meetings.
In short, use OOF with discretion. Period.