I may be the only person who thinks that the Really Big Sword is kind of good. I'll frequently take it with melee-centric classes as a hedge against mid-to-high level goo blobs. The deal with the RBS is that, assuming optimal play, its penalty is meaningless until reach the popcorn phase of the game (ie the very end). At that point, you can just convert it, having taken no extra damage and having gotten rid of those pesky level 9 goo blobs for a fraction of the resources you would have otherwise spent.
Ironically, the RBS becomes more cumbersome when the boss itself has high physical resistances. In those cases, you can't just convert the sword, and you have to content yourself with getting in one less hit per ding on the boss so you can soak the extra popcorn damage.