My thoughts on cleanses are they're not really cleaning your body as much as its cleaning your diet. I agree in the idea that the body is "self-cleaning" however it would be a good way to help if you've fallen into binges/overeating/holidays. I have the worst times with celebratory things as my friends and family seem to always be celebrating something and they do it with food. I have learned the hard way that I can't eat heavy wheat based foods so pasta is out as are breads and cakes. I also end up with stomach issues from high fructose corn syrup, simple sugars, and some sugar alcohols. It may taste good sure, and it may feel like participating at the time and "not eating like a bird" but man I pay for it later. I personally just up my veggie intake and eat more fish - no idea why but it makes me feel better if I do that a few days post party/holiday.
I'd like to do a juice "cleanse" to really pull myself away from all things processed, I just have a crap blender and don't have the money for a new one, let alone a juicer as well. While I'm trying to move away from all processed food, its a struggle. I think "cleanses" or juice based diets for a period of time can help with breaking the feelings that come with over indulging. It can also help you with correcting your taste as processed food are sweeter or saltier then their natural counterparts.
Just thought I'd give my two cents - I'm still learning myself.