Have to agree, can't see any good reason for not doing that.
Not to be picky, but not being able to have the tabs on the title bar above the address field is probably the main reason for not making IE9 to my default browser.
The irony is that the dedicated title bar they added below the address field (taking up extra space) is probably targeted for heavy browser users that most likely care. And the default setup that has the largest browsing area is for the mainstream users(that already have 3 toolbars installed taking up half the screen and don't even notice it). Pretty sure they will need to change it in some future release