The idea that everything in nature 'has a purpose' is a bit of a mis-nomer.
According to evolutionary theory, life will evolve over time if given environmental pressure to do so. This allows life to survive in slightly more hostile environments with each succeeding generation(provided the right random mutations), if a pressure to move out of the current environment is present.
The world does not, in fact, require that everything has a purpose. However, given the amount of time that life has had to evolvle on Earth, it has found it's ways into most environments around the world.
As the producer/preditor role evolved alongside, the crossing of environments became commonplace. Land preditors often rely on water-based food; the same water-based preditor may rely on photo-based producers and chem-based producers.
Thus, we get the idea that messing with the ocean can effect bears, etc. the "everything has a place" theory.
But infact, its not that everything has a place by some sort of universal law; its just that most things have come to be integral parts of the web, due to life evolutionarily modifying itself to use nearly everything it encounters.
In this case, oil as an environment is not one that has naturally allowed much to evolve to survive in it - IMO, due to both the harshness of the substance, and to the lack of surface oil that is readily approached by new life. It doesn't provide sustinance for a naturally occuring life form (yet), and so its presence doesn't have a direct effect on the environment.
Similarly, a substrata-pocket of pure hydrocholric acid does not provide a "useful" purpose environmentally - nothing depends on things that live there (because nothing lives there). However, usefullness in terms of geological preperation (ie, in the turning of stone to soil) may still be an indirect factor in the world ecology; for both a pocket of acid, and a pocket of oil.
There are bacteria who live on crusty caps of industria strength anti-bacterial bottles. If a consumer evolved to use those bacteria as a food source, and that consumer ended up being an additional food source to an animal already in the full food web, then the bacteria suddenly has become a part of a system it once wasn't.
I bet that even if we hadn't created bacteria which uses oil as a food (to clean up oil spills), nature would have evolved one over a long enough time and long enough exposure to oil.
Heck, nature figured out a way to directly dissolve rock:
http://www.sfgate.com/cgi-bin/artic...ive/2004/02/02/MNG874MPMM1.DTL&type=printable