Spirit Blade hits in a wide arc hitting multiple enemies, destroys barriers and guard faster then any other class and ignores enemy armor. The damage is atleast average.
Spirit Blade hits for like 500-700 damage per hit on mobs without spirit vulnerability so at best it's like a normal swing of a 2H sword. I would say it's VERY average damage and guard breaking with a warrior is better shield bash and take out the whole bar in one hit as others have said.
You might consider that damage a lot for a spammable ability however there are countless abilities in the game that are able to wipe the floor with every single mob in 1/5 of the time a knight enchanter spamming Spirit Blade would. Fire Mine, Wrath of Heaven+Spell Purge, Hidden Blades from Stealth.
Oh and the other spammable spec the Reaver hits for over 2.6k per swing of Dragon's Rage in a wide arc that of course decimates guard since you are a warrior. In comparison to that, the 500 or even 1000 damage of the spirit blade seems below average. I could see it if you said something about Chaotic Focus + Fire Mine spec of the KE, or the defense but, spirit blade damage? come on guys.