Hello!
  I'm glad, you have turned attention to this problem. :)

  You are saying there are some sort of "not very smart" users who may easily be confused by composing a massage to offline contact with a Chat window, am I right?
  Well, maybe, you are right. To be honest, I haven't seen such users yet, but I can imagine there are some. In such case, I suggest to make this feature OPTIONAL. I think that if those users are not very smart, they will not also change any default settings. There is no such a guarantee, of course, but I just think so. That is why making the current behaviour (composing Chat message in a Chat window and a Normal message - in a e-mail style window) default will save them from such a confusion. But we could discuss this question further and produce some kind of compromise solution. 
  The thing is that now I'm talking from the point of view of a regular user. And from this point of view it is inconvenient, when, for example the following situation occurs:
  Imagine that I'm having a chat conversation with somebody. We are talking and talking and then I need to go away from my PC for some reason. The person I was talking with sends me a message and waits for reply, but I'm away and I cannot answer. After a while, I come back and see that that person has left the network. Just went off-line. But I still have and un-answered message in my chat window, which I cannot answer using the same chat window. Instead, I must use a separate window to compose and answer, which, by the way, will not be saved into history. But this message is still a chat message and it relates to what has been written above. Don't you think this is not very convenient?

  I can also give another example. It happens to me even more often.
  Example: I write a message to somebody. He is offline, so I have to use an e-mail-style compose window and send a "Normal" message. After some days has passed, this person appears on-line and answers to me. But I just do not remember, what I had written him/her several days ago. In such case I usually ask to give me my last message fo I could figure out what we are talking about.
  This happens to me very often, I must say. It really annoys.

  That is why I wish there were such an option to make all of my messages (both incoming and outgoing) chat-messages. They are always saved into history and they are always easier to answer to. It seems to be not a bad idea to place such option somewhere between the settings. :)

  And talking about "standards". Well, I do agree that following standards isn't a bad idea in general. But I also do believe that the program must first of all be convenient to users, and follow the standards just after that. I also do believe that standards should be convenient for users too. :)
  Or maybe it would be better to say, that a program must follow the users' wishes where it would not be a too big violation of standards. Talking about Coccinella, it may be in form of not "saving Normal messages into history", but "converting Normal messages into Chat messages and saving those Chat messages into history" (of course, if the user turns this option on by his own hand). I guess, this wouldn't be a violation of standards. Or am I wrong?

  Anyway, I will use Coccinella with or without this feature, but I would like to make my favourite messenger as good as possible. :) Too bad, I'm not familiar with Tcl/Tk. :(

  Well, thanks for your attention. I'm open for discussion.
  Regards,
  Vladimir.