I think I see what you're saying but your screen shot cuts off the area I was talking about.
With Messages, there can be multiple accounts, as you know. with the new Messages, you don't really need a buddy list if you know who you are trying to talk to. You should consider Apple trying to think outside the box by introducing this new chat box, and also getting rid of the need of a buddy list.
Everything one can do with a buddy list, one can do with the "Messages Message Window" (sic)
Since your screen shot explicitly left out the part I was talking about, I'll post it here.

here you see I'm logged into FOUR different accounts (Aim, Facebook, Gtalk, and and AIM) but since I can have a conversation with someone who also has multiple accounts, the text field box indicated from which specific account I am on at the time. if you do not begin a conversation with someone, this area will be blank too, just like the video chat button. If this text box says iMessages, the Vid Chat button 
takes on a different form (facetime) then if the text box says AIM, GTalk, (etc.) where pressing it would give you the option to screen share, Video Chat (with photobooth effects), and audio chat.
What I mean to point out is, you shouldn't get too attached to the buddy list, and you should think of Contacts. app as your new buddy list, and the To field

As the way to access your buddies in Messages.
ever since messages beta came out I've been actively adding my friend's information to their contacts card for Facebook, gmail and Aim, and even twitter. I have never had a more up to date Address Book(Contacts) in my life! This is, what I think apple's ultimate goal is. To make you use your contacts.app and not a buddy list. A place where you can house all the information and take it with you (not just leave it on your computer). Some people may already have their's setup like this without needing to (you need to now, with the new OSX to see how it is all supposed to work in unison).
So if I have a friend whom has Gtalk and Facebook, and I add their information into one contact card along with their email and phone number. Any conversation I have with them through Messages will remain in one chat window and not dispersed between multiple sessions. At any moment a conversation can switch from being on facebook, gtalk, imessages (etc) by the recipient switching it or you switching it by clicking your contact's name in the To field and choosing their other accounts that you have tied to their contact card in Contacts or by sending a message to your friend from that account. The Text Box and a line break in the conversation will indicate the switch. No more having to sift through four windows of conversations with one person. it's all in one continues chat.
Try not using/relying on your buddy list. One day it may be gone.
This has been a PSA from your friendly fellow user.