I'm rocking a Fire Black Knight Greatsword+4.
The loss in physical damage is less than the additional gain to fire damage you get from infusing. A regular Black Knight Greatsword has C, E, C scaling in Str, Dex, and Faith. After infusing with fire it still has C,E,C scaling so no big loss there.
Not to mention it's got Greatsword reach with Claymore swings. I love it!