razz, I think that varies. If the obit is pretty straightforward, filling out the form does the job and makes it easier on the bereaved. But some folks like to tell a story or are particular about the light in which their loved one is put (not implying that you didn't; it's just that tellling the story is really important to some folks). That's when they should write the obit themselves.
Last month a cousin of mine (whom I did not know well) died, primarily of system shutdown caused by alcohol abuse. Her siblings chose to write an obit that carefully avoided mention of that and concentrated on the more functional parts of her life, which a funeral director would not have known. And the local paper charged by the word for it. I think the sibs viewed it as part of their healing process, so it was worth it.