That said, a lot of people share their accounts anyway. They may give their account information out to guild members in case someone needs a trade skill combine only they can do, or their class is needed for a raid and they can't make it that night. They may give the account information out to friends and ask them to grind honor or go on an instance run while they're at work. They may be siblings who share an account and split the subscription costs. They may be Arena buddies. There's plenty of reasons people share accounts, even if it is against the EULA.
Unfortunately, it seems like it leads to excuses as well. I heard the other day of a situation similar to the one Matthew Rossi told us about yesterday. Like in his situation, there was a PuG group. The dungeon in question, rather than Hellfire Ramparts, was Karazhan. The group in question had a Rogue who wanted a specific drop, in this case, one from the Chess event. When that item did not drop, he became angry. He ran further into the instance, pulled a bunch of mobs on the group, vanished, and left.
Naturally, the leader of the group was a bit taken aback by this action, and thus put out a warning on the forums for people to beware of the Rogue in question. one of the Rogue's friends immediately jumped to his defense, saying, "That doesn't sound like something [that guy] would do. Before you go around bad mouthing him, you should make sure it wasn't a family member or friend playing on that character."
Now honestly, that doesn't sit well with me. Even if you ignore the fact that the EULA forbids a friend or family member from playing the account (and most people do), there's a lot of responsibility that still has to go toward the player of the character for what other people are doing with the character.
Firstly, we have to assume, if we're not told, that one person is playing one character. It's not really default to expect that different people will play the same character at different times, especially not in a PuG environment as in the example above. Because of this, we assume any actions taken are being taken by the person regularly attached to that character, and for our own safety and for the sake of our repair bills, and we are perfectly justified in avoiding the person in the future.
It's poor form to chastise someone for assuming that one character is played by one person. One person playing one character is the norm, and what we are justified in expecting. Rather, it is the responsibility of the current player of the character to advise others that they are not the normal player of the character, or, at least to conduct themselves in such a manner so as to not tarnish that character's reputation and thereby the reputation of the assumed player behind it.
Secondly, the player of the character still has a responsibility for the character, since he gave others access to it. If you know your friend Rob has a tendency to be a bit of a jerk, and you give him control of the character, you're knowingly letting him use what reputation you have. If he uses it up, you probably should have seen it coming, and you should certainly realize that you enabled him to come in and be a jerk or break the rules, or whatever else he did. Blizzard certainly recognizes this. You may recall the story of Sinther of Stormscale. His account was banned for win trading even though it was his friends who used his account to do it, and not him directly.
For me, I've already heard the "It wasn't me, it was my friend/brother/sister/pet parakeet" argument quite a few times. I'm not sure anymore what's true and what's not. I'm beginning to suspect that some people try to excuse their own momentary breakdowns by pretending that it was someone else. Unfortunately, not even Shaggy can help you here, at least not in my neighborhood. If you won't at least acknowledge that you messed up by giving your account information to someone who misused it and promise to cut off that person so that it never happens again, I'll probably not care who was playing your character at the time. If I know that your character can turn from Dr. Jekyll into Mr. Hyde at the drop of a hat, I don't care why it is, I'll just avoid you and save myself another raid in which your character wipes us over loot.