The benzene does not come from corexit, it comes from the oil itself, please have a look at the components of corexit, posted on this forum, a couple of times.
So to answer your question, I don't believe the benzene is coming from the corexit, I think it is coming from the oil that was poured out into the Gulf waters. I also think that we need to be aware of all sources of benzene in our environments, and take measures to limit our exposure, even if we don't live in the Gulf area.
Please do not think that I approve of the use of corexit, I do not, I personally think there are better methods available to deal with oil spills.