A lot of people are saying that add-ons are making the game much easier these days.
As far as I know add-ons aren't allowed on arena tournaments.
And from my own point of view, wow is a game and what makes the game fun a creating the rules of that game. You don't create special things to, let's say, remind you to say "UNO!" when playing uno. Or, let's say, a system that would place a house at every street you want AT ONCE in monopoly. I know add-ons don't affect game play, but they do affect general game enjoyment and play-style quite a lot.
Now macros are awesome and in my opinion that's enough.