It's a weapon.  If you're going to be swinging it in an effort to kill enemies, then I'd say that putting extra sources of damage on it is a Good Thing(tm).

It takes a lot of work to get extra damage onto your character in non-weapon slots.  Why waste an opportunity to increase your damage dealt?

Also, in U9, you'll be able to craft a 33% fire absorbtion item (trinket, if I recall) and you can UMD a Fire Shield scroll, so you'll already be taking only 33% of the damage that you "should" be taking from fire.

You can drop that 33% down to 26% by crafting a 10/15% fire absorbtion weapon if you really want to avoid fire damage in one particular quest without spending any large Shroud ingredients.  That goes down to 20.5% incoming fire damage if you spend the 12 larges to turn that Shroud item into a 10/15/20% fire absorbtion weapon.

I'd say that taking 33% of incoming Fire damage from a crafted Firestorm Greaves style trinket + Fire Shield should be plenty, though.