The purpose of the obliterator is to get rid of all those bukeperries, old half-used torches, scrap pants, greyling trophies etc that you no longer want or never wanted rather than having to just dump them somewhere and hope you dont accidentally pick them up again later (you probably will). The coal is just a side bonus, although I'm pretty sure the ratios of stuff to coal people have mentioned are way off.

Regardless of what the ratio actually is, its a decent source of coal if you can grow crops efficiently or you have a grey dwarf farm.