First off, you can no longer interact with the command block's output feed. You can click on it and make your little cursor appear. but you can't click-and-drag to select data and the arrow keys don't allow you to navigate it either. 

Secondly, the input window doesn't scroll from side to side when making a text selection. The cursor and the end of the highlight just go off the visible section and it makes working with command blocks cripplingly difficult.