Recently, it came to light that one of our members was trading account information with another player on Alexstrasza. They were PvPing together and wanted to check out each other's toons or something like that. No big deal, right? No, it's totally wrong, actually, but I'll get into that in a sec.
Our GMs decided to keep the unguilded player out of guild chat by downgrading the account's guild rank while he was playing it. The player reacted in a manner that was beyond immature, totally insulting the officers and making sexually explicit and disgusting comments about our female GM. That resulted in a totally warranted /gkick of the account he was borrowing (to let the owner know just how bad the behavior was), which led to even more ridiculous and explicit statements, justifying the decisions of our GMs completely. That went on for a while, and we really couldn't do anything about it (I'll get into that in a sec as well) when we were finally able to get in touch with the toon's owner and got everything taken care of. It was a minor but important situation, which I think warrants a discussion on the topic.
So why is it such a big deal to be sharing your account information, and why would any respectable guild react in the way ours did?
1) Terms of Service - It'll get you banned, permanently. Good knowing ya.
2) Privacy - Your account information is now out in the open. Grats. Also, how do you know the other player doesn't have a key logger installed on his computer? Do you really want to login to a naked toon and an empty bank to find out for sure?
3) Reputation - If you're not perm-banned and your account gets hacked at some point or another, you will not have your items returned to you. Yes, Bliz keeps track, and you only have rep to lose. Also, if you're a nice guy and the person that you are sharing your account with is a douche bag, guess what? You are now a douche bag. And, we all know how non-respect-worthy players are treated in-game, don't we.
4) Accountability - You are held responsible for whatever action the other player takes. The other player gets reported, you get in-trouble and quite possibly perm-banned if Bliz notices the account sharing (this is why we couldn't report the retarded behavior we were seeing). It's not worth the risk, especially if you don't personally know and trust the person completely.
5) Dependability - if you are known to be sharing your account, you will be less likely to be depended on. How are we to know who's playing your toon at any given time?
6) It Can Affect your Guild and it's Rep - If you do get caught account-sharing or the person playing your account gets reported and you get banned, your guild is out a member (and whatever class, professions, and other uses you provide). Horde-side on Alexstrasza, that's a pretty serious issue. Also, any single member holds a great amount of weight in determining his/her guild's rep. If the player is abusing your toon and giving you and your guild a poor name, it can affect recruitment and the treatment other guild members receive by other players on the realm. Again, any self-respecting guild frowns upon account sharing for just that reason, so don't expect it to be taken lightly (given these reasons and the behaviors we were witnessing, it should be perfectly understandable why our member's guild tag was removed when none of us could be held responsible for the ridiculous behavior of the person playing his toon).
I'm sure there are many other reasons why you shouldn't be account sharing that I didn't go over. If you feel like something should have been covered, please feel free to comment and let me know what else you believe should be on the list. If you disagree with any reasons listed or feel that account sharing is perfectly acceptable, let me know as well. I promise you that I'll argue about it. ;)