For disabled users backspace to go back is tremendously useful. On keyboards with Alt Gr, pressing Alt + Left Arrow requires pressing down keys with both the left and right hand simultaneously. It's not possible if one of your arms is paralyzed. Targetting the back button (which btw, is getting smaller and smaller i newer browsers) with the mouse to click can also be a very time consuming task if you are suffering from involuntary shakes.

Some people actually surf using tab to focus links, then click with return and goes back with backspace. For them removing the backspace as back function is a big step backwards (haha) in usability. Yes, one could argue that they should just install vimperator and some other addons to make the browser usable with their particular disability. But most handicapped users are just as clueless as normal ones and have no idea how to customize their software.