A lot of these vegan diets were influenced by The China Study. Basically, it was a long-term study that found that people that ate significantly less meat and dairy lived longer healthier lives with less heart disease and cancer. The level of animal products that were consumed by the generally poor peasants in the study was about 10% of their diet, and they prospered health-wise when compared to our western diet.
Present day diet doctor gurus have extrapolated that if the people studied did that well on 10%, that 0% animal products might be even better. And some like Ornish/Pritikin found it to be relatively true for people with significant health considerations. Some have taken it even further. Or at least that is my understanding of it. This also often ties in well with people who do not eat meat for philosophical reasons. Vegan eating is also a place for some individuals with eating disorders to blend in well - the list of what can't be consumed is very long.
Others are coming to believe that there is possibly another unofficial condition/eating disorder - orthorexia - which is beginning to surface in people who are overly concerned about 'healthy eating' to the point of excluding more and more from their diets because it has been processed or is 'unnatural', or animal based, or not raw, or not organic, or tainted in some way. In my reading here and there about these 'plant-strong' diets, I have come to believe that is true.
That does not mean I think these diets are not good for one, because I do. And why I am eating this way myself. I like what eating far more fruits and vegetables is doing for me. The list of benefits I see in myself is getting longer almost every week. But... as I said above, and others have said, if the food doesnt taste good, I simply will not stick with it. And 'tasting good' to me includes some animal products including meat, cheese, fish, and even eggs if I want. Far, far less than before, but still some.
As to servings of vegetables, I am striving to eat as many as I can every day. More than the recommended 5. I want the veggies/greens to be my primary consideration, as well as legumes (beans). The rest is just flavoring to make the veggies very appealing in various gruel-type dishes (soups, 'stews', etc).
I also eat more oil than most advise. I want to eat more, larger salads, so in addition to avocado, I use a measured amount of extra virgin olive oil (1T) in a large salad. In the past I used to just pour from the bottle, and it must have been at least 3T/salad.
I am going for Dr Furhman's pound of raw, and pound of cooked, and since I am a volume eater, I usually hit the mark. I also am going for the onions, mushrooms, and beans he suggests. I also want to eat more fruits, but often simply forget to eat them.
My eating style is still evolving, but I do like the direction it's going, though it's not where I want it yet. Right now I'm just hoping to make it through the holidays relatively unscathed.