As a person who habitualy closes everything, I often average around 5 tabs open, rarely going above 10. With so few open at a time, the giant column of vertical space for the tab tree would be a waste of screen real estate, which can be better utilized to open another window i a vertical split arangement, or just for the website's content. Having said that, I know one of my friends has always at minimum 100 tabs open at any time, so he jumped at this feature as soon as ut released. It's a matter of preference I'd say.