I'm not a gmail dev and am not shy of criticizing Google when warranted, but I actually like the compose box. My understanding of the purpose of it is that it allows you to read email in a single browser window while having a compose window open simultaneously. I've found this is helpful when I need to copy/paste addresses or other information from existing emails into a new email. In the past I've previously had to have multiple windows open to do that.

I can see how it is a personal preference issue but it does server a useful purpose as far as I can tell.