I don't think that size zero models are the only reason for the increase in eating disorders etc, but seeing such images could contribute to more impressionable people having a skewed image in their head of what a healthy shape is.
I think that what we do need, is more of a range of sizes seen on catwalks and magazines. I know that designers say that clothes look better on a slimmer frame, but I don't think this is a good enough excuse to me.
Part of the problem I see is a real contradiction in the way the media (especially womens' magazines deal with weight issues. You can have a new 'fad' diet alongside pictures of a famous woman, saying that she looks good now she is curvy. They use size zero models, but tell is we should be happy as we are.
I don't think we should ban them from the catwalk, although many of them do not look 'healthy'. I'm sure there are women of this size who are naturally that slim, but not many. Lots of these very thin models are so young and perhaps not yet aware of the damage they could do to their health and future fertility.
But then again, doesn't measuring a model to see if she is too thin, just as degrading and worrying as measuring her and saying she is too large? There are no easy answers.