I,myself,am not a vegan(yet),but I highly respect that lifestyle(along with vegetarian),and have many arguments as to why humans should not eat meat.
Of course,as you said,it's not the lifestyle for everyone since the amount of carbon-hydrates,proteins etc is much smaller than that in animal products(although there are no animal-specific vitamins,minerals,carbon-hydrates etc).But if you can do it,there really is no reason why you shouldn't do it.