Hi, today, Plus! suggested me to turn on tabs, I've tried it and it seems very nice. I've choosed tabs to be placed on the left, but I am very unpleased with the waste of the space. Could tabs occupy only the space they need to? (as on the screenshot)

No its not possible because of the way its actually designed. The tab bar is actually hooked into the window of the conversation therefore the bar is the size of the height of the window.

Well...not surprisingly..... Is that so bad suggestion?

I dont think it would be taken into consideration because what you have to think about is what happens when the border is shown (so you see the Menu and the Titlebar) then you have this gap in the window that takes on anything that is moved over the blank area (so it leaves in essance residue from the other windows so you would still see them, kind of confusing not sure if it makes sense).

Anyways I don't think its worthwhile doing.

It is a good suggestion, however, it just can't be done technically right now. This could be an idea for Windows Live Messener though as its contact list already works that way.