Comment 61 for bug 432814

David, please consider the following: nearly all touchpads have hardware buttons for right-click. Nearly none of them has a hardware button for middle-click. It takes an unhuman skill to perform a 3-finger tap, so with 2-finger tap assigned to right-button, the only usable way to perform a middle click is to plug in a mouse. Middle clicks are of great use in Linux (clipboard insertion to name one of the essential things), so having no way to do them is a bad bad choise.

Anyway, that's not what the bug was about. The bug was about the fact that there was no way to change the default behaviour at all. Now there is a way (or will be, as soon as Alberto's upload builds), and that's the fix. What the default setting should be is another topic altogether.