This suggestion gets brought up pretty often, mostly ever since they did this in PvP. The problem with this is not all classes have a basic 1,2,3 combo. There are classes with branching combos that such a set up wouldn't work for at all. This would be all but impossible with the current design of Monk with Perfect Balance and Form Shift, not to mention that you can swap which part of the combo you are doing at any point with monk and sometimes have to. Then there are classes that are proc based that you wouldn't be able to do this with either like Dancer and Bard, or any of the casters really.