Not sure how many buttons most people have on mouses, but many gaming mouses can even have 8 buttons. As left-handed I would like to use tablet on left- and mouse on right hand simultaneously by using custom keybinds. Stuff like buttons 4-5 working as undo/redo could maybe even work as default behavior.
Can't you configure your mouse to send key events?
Yep, on windows :p my mouse drivers are not supported on linux so no hardware rebinding for me. (Without reinstalling os for it that is)
Do those buttons send any event at all on Linux?
They have normal functionality in linux, such as in FF going back/forward pages (buttons 4-5) and what ever software has binded other mouse events to do. I use these often when gaming especially for MMos.
Related: bug 402544