I believe that if you queue with a group of 3+ people, the loot system becomes bugged and you can need roll on items that you otherwise shouldn't be able to.
However, I noticed last night that in a 5/5 guild group I was unable to need roll on Plate, Mail or Cloth items.
And yes, if you are turning in a quest as a dps agility spec it will typically only show you the agility mail gear. If you respec before turning in to a caster spec you will see the intellect options. I did notice that sometimes certain quests show all rewards (agility and int in my case) but those are far and few and don't apply to the dungeon quests :)