I agree. With juice fasting the body is in a detox and healing mode without the distraction of energy-sapping digestive processes. It can draw on a wide variety of nutrients very fast giving the body the support it needs. Personally, I prefer to also supplement with some green powders and other fibre to keep the bowels open and help the body eliminate toxins, parasites and heavy metals etc. during the cleanse. Yes there are some sugars, but minimal if you use as you should during a juice fast, mainly vegetables, but unlike the pre-packed juice you buy in the stores, it's fresh juice full of nutrition and live enzymes.
This, as you say, is not about going 'vegan' or vegetarian, it's about doing periodic housekeeping.
Just check out Joe Cross and his 60 day juicing regime. Whilst he now eats more raw veg than he used to, he is not averse to an occasional juicy steak!
I have moved on from the idea of extremes, to being more balanced between both paleolithic and veggie. They both have their merits. I don't think it is good to go to the extreme in either direction. Its not a competition! As Dr. Campbell-McBride points out, many ancient cultures would ferment plants and seeds to make more easily digestible and highly nutritious beverages and foods.
Protein and fats are building blocks, plants are healing and detoxifying. We need the aspects of both.