It's pretty simple: You need a dps in your duo for fast leveling, you need a tank/healer combo for tough fights (champions, Heroic2, some class quests, or when you are close to falling asleep). So, as a rule of thumb: healers spec should go gault/torian most of the time, switching for skadge/blizz when needed; tanks should go gault/torian most of the time, Mako when need arises; Dps spec should prolly go mako all the way.