I have to echo Gigi's reasons, BG. I have a lot of anecdotal information (beginning with my own story), but the cases cover such a broad range. It strikes me that I know of very few people who have refused treatment (beyond surgery); I know of many survivors who've gone through radiation and chemo and apparently won, and then a few who suffered through treatment and lost, which always seems hard, but it is a choice that most people seem to want to have available.
June Callwood is an example of someone who refused further treatment and then went on to live, I believe, ten years (?) beyond what had been predicted -- and she lived them very well until almost the end. I suspect that's a very unusual case, though.
I got the luckiest diagnosis -- surgery with no further treatment recommended, although regular visits to the oncologist. That's also very unusual, though. And you're always told, once a cancer patient, always a cancer patient.