Between Fire and Fighting there's 40 physical and 23 special, so I do see the point in focusing on physical offense,
but Fire seriously leans towards special, and as far as effectiveness goes, it's a lot more dangerous than Fighting.
Or is the point of it more that, with x4 damage, no amount of Sp. Defense can really make up for the Fire weakness in the first place?
Edit: Okay, so it definitely should still die instantly from Fire moves even with higher-than-average Sp. Defense?