Why be well-rounded?
Well-roundedness for undergraduates is partly about being adaptable and therefore prepared for a college education. Someone who both has good grades and also competes in athletics, holds a job, etc. has perhaps shown some experience in balancing their different responsibilities. Additionally, someone with a breadth of experience is more likely to actually know what they want to do next rather than sort of just blindly going with the flow.
Usefulness of well-rounded experiences for potential graduate students
I think this last part applies best to the appropriateness of well-roundedness for graduate school. Someone who has shown multiple interests and is now choosing to specialize one is probably making a more informed decision than someone who hasn't. As an example, if they've gathered experience in math, physics, and engineering and are now applying to a physics graduate program, they probably know physics is actually the field that excites them most. They also might carry in some extra skills that not everyone in their graduate program would have: programs in the biological sciences or business, for example, can really benefit from incoming students with quantitative or computational skills.
Knowing what sort of recommendation letter to write
As far as how to write recommendation letters, I would recommend asking the student for some feedback. Prospective students should have some idea of why they are asking certain people for recommendations; when I applied, I asked for one professor who taught me in courses on the history of science, completely outside my field of interest. I hoped to convey some breadth of experience by asking for that letter.
My other recommendations were from people more in my field and were better positioned to talk about my potential for research (which is still the most important thing in a graduate application). If those people had written about breadth instead of their opinions of my research potential in my field, they would look quite odd and probably not be helpful.