Well, I do believe there is such a thing as a "well balanced diet" which includes food from all groups.
What are the food groups? What's the right proportion? Does it vary with age/ethnicity/nurture?
These are precisely the questions being examined, so telling people to eat a balanced diet without telling them what that actually is, doesn't help.
How are the food groups defined? Why isn't ethanol included as a food group? Do you consider fiber in its own group?
Taking "balanced" portions from a set of groups makes the context in which the groups are defined very important.