I agree, but I would go as far to say that if you did not have purpose (or a porpoise) your options would be limited only by your imagination (in an equal society).
If I honestly believed in a god and believed my purpose was to convert others into the fold then I would be doing that till I died. This would limit my ability to experience lifes vissisitudes. Would I be happy? Probably.
It seems to me that if you find what you like doing, do it. Who needs some god figure to sanction that choice? Our own morality should guide us. If it does not hurt any one else for you to say, study porpoises for the rest of your life, go for it.
It seems to me this purpose question is a red herring.