With a warrior tank I DPS trash down one by one. If it is non elite trash and they somehow attacked the healer and dps toons then I AoE. Four AoE will kill anything non elite really fast. I have found that burning down eliete trash is really fast when done one by one.

If you fight Novos the Summoner in the Nexus instance he summons adds right inside of all your dps+healer toons. AoE is good for this. There are a few situations where AoE is the best call. So you better have it handy for when you need it. Keseleth also summons adds that need to be AoE downed.

I have AoE setup 2 ways. 1 button for all AoE and round robin AoE. Sometimes I don't need all 4 AoE's going at once and its better to preserve mana. My setup allows me to AoE and still do single target dps.