Personal advantages of being a referee once you quit science?
To me, refereeing papers in subject X is a non-negligible part of being a professional academic in subject X.  So the premise that you want to referee a physics paper having "quit science" seems moderately self-contradictory: using your expert level qualifications to evaluate a scientific work is being a scientist, isn't it?  I'm almost sure it is. :)

So I guess you haven't "quit science" completely, or -- and this may come to the same thing -- the physics community is not fully aware of this fact.  One thing I might do if I were you is to make sure that the editor who asked you to review the paper knows that you are no longer employed as a physicist.  If I were an editor, that would be useful information, and unless I knew you rather well and knew that you retained a special expertise in this particular area, I would probably select someone else.  

I think it's clear that you no longer have any kind of ethical obligation or professional expectation of reviewing papers.  I also suspect that once the community understands that you really are doing something else now your referee requests will come rarely or cease altogether.  So this is probably a decision that you need to make more for any particular paper than as a general life decision.  I would say: if you feel just as qualified to referee the paper as before and if you want to referee the paper, then do it.  For instance, if you see something valuable in the paper that you think that some other qualified person might miss, then it might be a nice parting gift "to science" to speak out for the worth of this research.  On the other hand, if it's just business as usual (or worse, motivated by "the need to continuously publish"): well, you don't have to clean the bathrooms in the apartment that you're not renting anymore, do you?

If you have thoughts of breaking back into science later on, then I think you need to think seriously about how to keep your connections and your intellectual life, um, alive.  There are many aspects to this, and making sure that you continue to referee papers is not, so far as I can see, especially close to the top of the list.  If I'm looking at the CV of someone who left academia for a while and is trying to come back (and it does happen), I will look to the work they have done recently and their plans for work in the near future.  I don't really care whether they've refereed some papers in the meantime.