I think the roles have been put there to prevent class elitism. Say if the role of tank is put down, then Pallys, Warriors, and Druids can all be held in the same regard. But if people can specify needed classes then it will obviously hold a priority in people's minds when creating a raid. Naturally due to stereotypes the priority would be warrior, druid and the pally. For example, I hardly ever consider shadow priests, shamans, balance druids, and prot pallys (well maybe since the new patch) when forming groups. This means if I was to form up a raid on the planner using only class specifics some people in the guild would hardly get a looking.
on another note, are we still using the old planner for now or are we using the new one straight away?