Hey,
Lemme start of by saying that AoEing is GREAT for dungeons in general. However, there are a few exceptions to this rule. Deadmines is a generally HUGE exception.
1.) Tanks in Deadmines are an interesting bunch. New players tend to not realize that they are MUCH better of in Defensive Stance than they are in Battle Stance. This takes away their best tanking abilities. Even if they're not new, they only have so many abilities to pull hate off of you anyway. This gets much better in higher levels.
Moreover, if YOU have hate on you, then the healer has to go through much more mana to keep you alive.
2.) In a decent party in Deadmines, you shouldn't be fighting any more than 3 mobs at a time. AoEing 3 mobs at once is pretty good. AoEing one half dead mob and another mob is a bit wasteful.
AoEing less than 3 mobs actually hurts your DPS. You'll do far more damage by using single target spells AND save on mana. (Technically, you'd do more DPS to two mobs, buts its very likely one of them will come over and smack you before you're done casting. That means you'll be losing precious seconds on Blizzard.)
3.) Polymorph is your friend. More importantly, its the party's friend. You're in a pretty unique position. You're a solid damage dealer AND you have the ability to outright take a mob out of combat. Polymorph does wonders for adds and can seriously save the party from certain doom if used properly.
Now is this to say you should never AoE Deadmines? Hardly. If the circumstances are right, then by all means nuke away. Just remember that a lot of parties are made up of well-meaning but rather ill-informed newbies.
And I hope this doesn't sound like a rant, it wasn't intended as such. Just been through Deadmines a lot recently and have noticed a lot of this happening and helping to cause party wipes. Kinda funny to sit back and see how long it takes for the squishy Mage to go down after he takes hate away from 2 Elites plus 4 non-elite adds...