Fire/Ice would be effective on Grass (*4), Bug, Ground, Flying.
You lose the Super Effective on Ice, Steel, Dragon
--> Seriously not the best type combo ... I would prefer two different Attacks :D
Fighting / Ground would be more useful, with being effective on Normal, Steel(*4), Rock(*4), Ice, Electric, Fire
You only lose Poison ! A great combo :)
Anyways, yeah, GF probably doesn't want something that deals 8x damage from type matchup alone. As much hate as I have to Heatran, I can't imagine it eating 8x damage from Fighting/Ground attack in a go.