Comment 18 for bug 355340

Stylus and eraser are mapped by default to a left "click button" because even then applications can still assign them to two different actions (since they can distinguish between a pressure from the pen tip and one from the eraser, the same way they can distinguish a left click from a mouse and the "left click" from the pen tip - they don't all do that, but then it's a deficiency of the application, which is also there when you would want a mouse and the pen tip to be assigned to two different tools in a program). Assigning one of them to something different might not make sense since you then won't be able to use the eraser to "draw" or erase in some application, where the middle click has a specific action. In drawing programs like Gimp or Inkscape, using the middle mouse button you can't draw anything (for example on Inkscape using the middle mouse button drags the page, hardly what the eraser is made for).

The middle mouse button, like the right click, is better addressed by means that don't require you to flip the pen, since that would break your workflow. And the eraser is, in people's mind, there to erase (that's why it's put there at the opposite of the pen tip, even though it means you need to flip the pen to use it, instead of just addind another side button that modify the pen tip behaviour). Assigning it to the middle mouse button would mean it won't be able to erase (at least in most programs I can think of).