In reality, it's not.
Look at this episode. Had they used the firemaking challenge to break the tie, then no one would have a reason to scramble. They split the votes 5-5, and let the fire fall where it may.
But, because of the rock draw, it leads to everyone playing a massive game of chicken, hoping someone on the other side flips.

fire making is on final 4. They did it last season between Aubrey and Cydney, so it is still in the mix.

I think they only do that for when its close to the finals, like 5-6 left.