I think the reason people want to add macros to the action bar is for multi line macros. For example, you can have the button for Banish II on the bar, and it offers you to the option for AoE when you press it, but as a macro, you can have it as a one button cast, and also add in any gear swaps, /p lines and use the /aoe on /aoe off command lines, so it's all under one keypress, but this opens a door for exploitation too, as people could automate actions by doing this.