A paint roller is a useful tool for painting large areas, such as walls.
A paint brush is used for smaller projects that require more care and attention to detail.You shouldn't paint a portrait with a roller. It would look terrible if you tried. Someone who paints a portrait with a roller does so because he lacks respect for his subject. Doesn't care to study the subject and learn its details. Is unconcerned with how the subject will react to the portrait.
A person who paints a portrait with a roller also lacks respect for his audience. He assumes his audience lacks the intelligence to see the difference between a paint brush and a roller.
An example of painting with too broad a brush: The Difference between Protestant and Catholic Christians.
Addendum: You can read the whole of the article here: Why Catholics and Protestants Don't See Eye to Eye. (Thanks, Elena!)