If the person isn't a complete idiot, that one hit can cost you the game. If you're going to run back, depending on the angle they can still kill you. (Unless you're like on flatlands, lol)
Anyway.. I never find myself with a small amount of iron ingots or FnS'. If you aren't completely aggressive and go on a rampage after you get a sword you should probably have a decent chest route.. Which should probably get you one FnS at the least, one is enough generally. Fire spamming will generally be less beneficial to the person that did it.
Summing up, if you need that many FnS' to negate making an iron sword you're probably clueless on how to use a FnS efficiently or just love spamming fire xD
Some people just want to watch the world burn.