I agree that -your character- should never go out of character. But to say that you can only talk to others in character is completely ridiculous. On occasion, it is necessary to [[ooc]] with more than one person for multiple reasons. One is exploring a bug. Another is clarifying something your character is doing. [[ooc]] is used quite a bit for saying you have to leave, and when you may be back, followed by the [[ooc]] farewells and goodwishing. This is ooc, but in a good way. It helps to bind the PLAYERS behind the characters together in a positive way. It lets you feel that there is someone behind all those identical faces, which there are.
We are real people playing imaginary characters. And though I try to make those characters as real and believable as possible, they will never be real. To try to force folks into [[ic]] all the time is not only impossible in this situation, it is damaging to the feel of the game.
Often I use [ooc]] to catch up players who come into an RP in the middle and wish to join in. I tell them what their characters may know or see happening, and what the RP is aimed at. Then others players in the RP can do [[ooc]] of their own to further clarify the situation. This is out of politeness, as sometimes it takes quite some effort to get into and understand what is going on in an RP. It also tells newcomers that they are welcome to join in, but gives them a few guidelines as to the mood and focus of the RP. This is positive for both seasoned players -and- 'newbies'. There have been many times I have seen a new player come up to a good RP and just stand there watching for a long time, before running off. The ones I have [[ooc-ed]], either in chat or tell have expressed their feelings that they may not have been welcome, or feared that they may have ruined the RP.
Another place I use [[ooc]] is narration. This is to tell folks something they may notice about my character, or something he/she is RPing such as a light in the sky, or a note on the wall. Now this seems to be a hot spot among some players. There are those who like to use their imaginations to enrich the game...then there are those under the impression "If I can't see it ingame, or work under game mechanics, then it does not exist." In my opinion, -that- is [[ooc]]. For there are many things ingame that our characters know, have, or see that we, as players, can not. If we are to roleplay that they are alive, and have lives, we have to go beyond what -we- can see. Otherwise, all I see right now is hundreds of clones running about looking for the strongest weapons they can find.
So, though I am totally against using chat to talk about your day, events in RL, "how do i do this quest", or your l33t new sword, I see [[ooc]] as a useful tool for RP.
Now, /tell is a completely different story all together. Much of the time, I use it for talking to other players I know, or advising new players on good RPing. In fact, the only time I use /tell for RP is if I am standing close enough that someone could hear my character whisper, or if I pass them a note. Using /tell for anything other than those things would be [ooc].