Auto attack is basically for people who want to be doing something other than playing the game: watching TV, etc.

Auto attack has been starting to disappear from MMOs. While most still have it, it's useless. Examples:

-Aion - Useless since you're too busy spamming other skills
-Vindictus - 'nuff said
-DCUO - non-existent
-Rift - Useless since you're too busy spamming "combo builders" and then your "finisher".