You obviously aren't strong on reading. Guardians can spec into tank or dps and Clerics can spec into healing or dps. They can select either for those. Yes there is a point to this post.
No they really can't play other roles. Let's be real here. In high end game content, you won't be looking for a rogue to roll as a tank or rangers for healing. Maybe leveling up that might work. Please show me the copious amounts of tank builds for rogues and healing builds for rangers. You going to be looking for a rogue…
Exactly, as a GF it's irritating to go into a mob expecting some heals and the cleric doing nothing in that regard. If you're a cleric and don't plan on healing, but dpsing, it'd be nice to let the tank know.
You don't allow rogues and rangers to select tank or healer rolls. If there class is not designed for that, they can't select it. WoW has this system and it works fantastic.