1) We are used to have the toolbar at bottom. 2) This makes room for more icons. 3) It is better to lose some space horizontally because we have windows side by side horizontally, usually. Then, why a vertical toolbar?
Netbooks (or more generally: small screen devices) where you can more easily lose a few pixels from the side than the bottom.
Unity ruined Ubuntu on my netbook because of that. Sites that I read before no longer fit in the screen width, including Ubuntu's own homepage.
I dumped that noise and installed LXDE and made my netbook usable again.