I was watching "Supersize me" and seeing the awful effects eating fast food can have on your body. However, I think that purging fast food is more detrimental to your body then keeping fast food down. I think that is an important idea in recovery from purging behaviours. Accepting the fact that a fast food meal won't kill you, but purging might. Therefore, while in recovery, it may be necessary to have fast food more than an 'ideal' number of times, in order to practice being okay with eating and keeping down fast food. Not purging is a skill, and like any skill, you need practice.
With regards to binging behaviours, I know personally, that if I'm going to eat unhealthy food, I am going to binge on it. It's all or nothing in my mind. This isn't a healthy attitude. One should be able to have a 'cheat meal' without consuming 5000 calories. It's more detrimental to eat 20 tacos, then it is to eat 2 or 3. To get over this all or nothing thinking, it makes sense to eat fast food in moderation, in order to practice getting over this all or nothing thinking.
When it comes to restrictive behaviours, I think again that the main problem is the all or nothing thinking. Restricting fast food may not be unhealthy, but how do you draw the line between fast food and restaurant food? And then how do you draw the line between restaurant food and homemade restaurant style food? The idea is that restricting one thing can easily lead to restricting more and more foods when you have a restrictive eating disorder. Therefore, accepting that it is okay to eat fast food is crucial, in order to stop the cycle of restriction.
Now of course I'm not a specialist, these are just my thoughts that I developed through personal experience and having friends go through treatment. My main thought is that you may need to practice eating fast food so that you can choose healthier options without being driven by your eating disorder.
Here is the video: