Smite is for the battles when you are getting swarmed by a plethora of enemies... You can keep the away and then pound the crap out of another... Saervok fought his way throught Sendai Enclaves slave area with Smite on boy was it ever funny!
Greater Whirlwinds aren't that bad, but I prefer Smite... Of course, you can only get 1 of it.
GW is pretty snazy when only using 1 weapon I must say... especially any that can heal (Foebane, Black Razor ect).