I made a ticket [1] about exactly that with a mockup (which I seem to have deleted from my droplr over that time; though it looked very similar to how iMessages looks now) 2 years ago and the response to the idea was a "no". You should make a ticket yourselves and see if they've changed their minds yet.

I'd love to see a single window UI in Adium but just making the mockup and saying how good of an idea it is doesn't seem to be convincing enough for someone to actually tackle the project.

[1] https://trac.adium.im/ticket/14843