He he
I actually like the + button very much. I even had a skin for FF3 that enabled it.
The good thing about it: if you have many tabs open, it is still very easy to open a new tab. Without it, you will have a hard time double clicking that tiny empty space next to the tabs.
I know there are other way to open a new tab, but this is my favourite way.
I suppose if you're clicking to open a new tab it's useful. I assigned rotate clockwise to open a new tab via MultiClutch, so I use this or cmd+T.