I see nothing wrong with a child having a few cookies here and there or a serving of ice cream after dinner from time to time. Everything in moderation.
You can't teach a child how to control their urges for sweets and junk if it's not around.
Fast food should be used as a very special treat, but not daily or even weekly. Saying, "we got .Mc.Donalds tonight because .I was too busy to cook" in my opinion is a cop out. "we got .Mc.Donalds tonight because .I was too lazy to cook" is more like it.
Food should never be taken as a form of punishment. It's abuse.
I don't really see an issue with using food as a reward, as long as it's clear that is what you are doing. If you are teaching a child to use the potty, than giving him a handful of .M&.M's is ok. Giving a child a food reward for behaving in .Walmart is silly. There are so other ways to reward children, and many kids would rather hear their mommy or daddy say how proud they are for being so good and get a big hug, than a candy bar.