I think that feature would be great. Even better, moving the fixtures in real-time with the arrow keys, in edit window or against selected fixtures from the fixtures page. That would be wonderful when the singer steps aside for a while, or moves forward to the spectators.
For example ALT+Arrow keys to move selected fixtures, in relative mode, that is, moving step by step, preserving the spacing.
I had this feature in a very cheap console i was using, an SGM Studio 24. It had a joystick which would send absolute x-y coords, while the arrow keys would increase or decrease the x-y dmx values, one by one. That was really handy :).