Two reasons. First, when I accidentally map a shortcut from another command, it removes that shortcut from the first command and puts it on the second. Then I have to track down that first command and manually reassign it, I'd rather have the ability to just remove the shortcut from the second command I just made.
The second reason is because I think down the line, when more features are added, I think there will be a use case where the same keyboard shortcut can trigger multiple commands based on what is selected. For example: the "Trim in point to playhead" doesn't do anything if you have a transition selected instead of the clip; so if my other feature request was implemented that trims a transition to the playhead, then I could map the same key to that command as I do for "trim in point to playhead". This would allow commands that have similar concepts mapped to the same keys yet still only trigger independently based on what is selected. (This could introduce from bugs and problems if users didn't map things correctly though)
