(1) I could write several messages at once. Useful if you want to look things up, and keep multi-tasking.
(2) I can refer to other messages while writing one.
I can see how it is a personal preference issue but it does server a useful purpose as far as I can tell.
The new compose window makes me want to switch away from Gmail. It should be purely optional.
I just wish the compose box was freely movable. On a very large screen it is indeed awkward to type in the bottom right corner only.
Seconded. It's actually very painful to write on the bottom right corner; also at times pasting large amount of text in to the box screws its interface. When you've some formatting on the text pasted there, something goes crazy.
I recently quit accessing my primary Gmail account and this is one of the main reasons for it.
Other being the amount of spam that started coming in, in the name of notifications, deals and whatever "my friend has unlocked...".
I also found Gmail forcing a lot of G+ around the header of my inbox, and tonnes of services that I have never opened or would not be interested in. Unable to switch those off.
That said, I do find this label-tab option interesting. Will definitely give it a go.
[Edits] Added thoughts.