I don't think it particularly "dumbs down" the game where you don't have to press 1 2 3, but instead can just press 1 several times...

And it would indeed allow for more open slots on the action bars, although like I said, It'd be like 2 or 3 buttons for most classes...
But, MNK you can easily free up a total of 6 buttons, DRG could free up 8, and SAM that was brought up could 6 if you consolidate their 3 regular weaponskill combos and their 2 aoe combos...

And on the topic of opening up spots on action bars, it's always a mild worry around every expansion release how they'll manage to fit in all the new skills. With consolidating the combos to a single button, they would have less of that for the next expansion...