My mouse is actually critical to how I use i3 - I've got a lenovo thinkpad usb keyboard, with the left, middle, and right mouse buttons below the spacebar and the red nub for the pointer. I use focus-follow-mouse and when I need to change my active window I just flick the mouse over to where it needs to be - it's super fast. It also works really well with laptop trackpads and an actual mouse.

I know other window managers allow you to use focus-follow-mouse, but I haven't found any that make it nearly as effective as i3.

Would you mind elaborating on what use case you think a mouse could come in handy for when you would be using a tiling WM? I recently switched over to one and the only time I've used my mouse was when I had to click on something in Firefox.

The mouse could be convenient to resize or rearrange windows. For example, it would be nice if dragging one window onto another swapped them. I think that would be faster than swapping them with i3's shortcuts.

Alternatively you can use tridactyl to click on links with hint keys.