Well when you look at it, the tree bar is designed to manage more server windows and channel windows etc. than the switch bar. The tree bar can easily handle 2x or more windows than what the switch bar can, and with the collapsible branches it makes room for that much more.
I'm only on 3 servers with a total of 22 widows open through out. and the switch bar is probably 2/3 full. With the tree bar with all items displayed on the tree, it takes up a little over half of the bar.