Stack Overflow uniquely requires moderators to operate with a standard of duty. Even so, moderators generally feel a sense of obligation to be around in case of problems. Such voluntary dedication is much appreciated and often means people really enjoy what they are doing. But everyone needs a vacation from time to time.
Currently, we have no good way for moderators to communicate their vacation plans to either to Community Managers (we suggest they email us), other moderators (this usually happens in chat) or other users (more on this momentarily). In addition, the temptation remains to "just handle that one flag" if a moderator happens to browse their site while theoretically off-duty. Finally, I believe "vacation" might be a graceful way for moderators to step down if the job stops being fun and rewarding.
Pro