Well actually i coulnt agree with this less. I dont think the RoC campaign was even in the slightest bit like King Arthur.
For one thing Merlin was ARTHUR'S advisor and teacher. Jaina was neither to anyone. in fact she was just a student. Also, Jaina became a leader as time went on so if anything she could be better compared to King Arthur.
For another thing Arthas was trying to save his kingdom, not destroy or conquer it. everything he did was out of love for his people and the reason he fell was because his soul was stolen from him due to his lust for vengeance.
As for King Terenas being like Arthur, he was not a warrior, he was not wise (at least not as much as Arthur was) and he had no "merlin" in his life (unless you count Uther, however Arthas killed Uther so uther cant be compared to Merlin).
And on top of all that if you were to say someone was Merlin it would be Medivh the prophet, however he cant properly be considered Merlin because he helped out to many people instead of just 1 person and he wasnt anyone's mentor.