That's interesting. I practically never use the mouse to switch between windows in OS X or Windows.
The biggest problem I have with tmux is vertical splits don't work with native copy-paste because the selection goes across both panes. Using multiple terminal windows, on the other hand, doesn't have that issue.
Using a mouse, you can use tmux's new (in 1.8) pane expansion. prefix + z causes the current pane to take up the entire window. prefix + z again and it goes back to normal. Essentially, it's a maximized/restore capability.
The biggest problem I have with tmux is vertical splits don't work with native copy-paste because the selection goes across both panes. Using multiple terminal windows, on the other hand, doesn't have that issue.