Originally Posted by
Nobleshield
Although I argue for rotating, I can speak to the reason for this. It's because at the end of the day, your toon's progress is more important overall than your guild. What I mean is, if you are in a 13/13H guild but you happen to be 2/13H, then for all intents and purposes you are 2/13H. If you are sat on a boss, that's one less boss that you get to kill (at least reasonably, as it can be assumed at some point you'll be there for the kill) which means you're one more boss short of however many heroic bosses. Now if you are in a great guild and never have reason to leave, that's not a big deal. But, let's say that things go sour and you look to leave your great guild, or let's say for a nicer example that you get a new job/start a new semester and can't meet your current guild's raid schedule anymore, so you need to look elsewhere for progress.
You've been rotated in and out of a few fights because you're a nice guy like that and you want the guild to succeed so you don't mind. But now you're looking for another guild, and you're only 6/13H while your guild is 13/13H. You are 6/13H to other guilds, because nobody gives a shit what progress your guild is as that doesn't mean that you are as skilled (and can even work against you as the unasked question is "Why were YOU the one rotated out? Perhaps you were causing wipes on the fight"), so even if you were an amazing player you aren't presenting that to others because all you can say is "Well my guild is 13/13H but I'm only 6/13H because we rotate people in and out of fights as necessary".
That's one possible reason; it hurts you if you look for other guilds because you can't show the same level of progress as the guild, and it's your personal progress that really matters at the end of the day.