It states in the Revan novel that he is no combatant and that he was very easily disarmed by the Exile and Revan, he spent all his time learning arcane Sith secrets, he gave no time for Lightsaber combat.
However you say that he is utterly useless in combat with a lightsaber. I disagree because this implies that he is bad at using a lightsaber. I think the fact is is that his fighting style doesn't rely on the use of a lightsaber (kinda like the Consular compared to the Knight in Tor).
+ I don't think we really know how good he was with the blade.