Positive Platitudes for RL Vacationing
Outside of the positive health benefits of getting a little sun, there are (IMHO) a number of positive benefits to take a week off from the game.
A Time to Rest
While leveling up an alt from 0 to 85, personally, I like to take advantage of every +XP bonus available. The one bonus that Blizzard has had since early on is "Rested Bonus". Implemented as a means to limit game play, it also adds as a wonderfully nice +% XP gain for every honorable, monster killed. Running chained dungeons is a quick way to burn through your rested bonus gained over a single night. Let that one night stack to a long weekend or even a full week, and you could have a full level worth of Rested bonus to burn through.
Wrong Kind of Crowd
PUGs are inevitable in this game. Leveling up a toon via the LFG tool, you can easily hit the same group of people night after night. That may or may not be a good thing. Take a few days off and that group of people will level on passed you. Leaving you with a completely new group of players to complain about. That DK tank who doesn't seem to be able to hold aggro? Gone! The hunter who seemed to 'fall asleep' at the entrance portal after clicking 'accept' into the dungeon. Gone!
Bad Options Don't Make Good Job Security
This goes for raiding. If a raid group feels that they don't have anyone else to fill your shoes, they'll just sideline and never progress; other guildies who are willing, may never get asked to participate. The guild will never progress, and many players may eventually quit, looking for more reliable raiding options. Don't make yourself too secure, be flexible and make sure your guild knows. You could even suggest a substitute DPS while on vacation?
Putting Your 'Family' First
You Deserve it.
Overall, take time off. Don't feel obligated to pick a vacation spot just because it has a good Internet connection. Don't succumb to your raid leaders request to purchase more RAM for your laptop, just so you can run your raiding addons. Don't turn down that invite to hang out with your family during the fireworks display, simply because it overlaps the guilds hundredth attempt on that X of Y raid boss.