If the writer's opinions are not objective, the source:

has authority

is not accurate

is biased

lacks currency

If the writer’s opinion is not objective, it mostly means that the source is biased. Objectivity is very important aspect of any writing. If the readers perceive that the writer is being biased, they probably will undergo selective perception and chose one thing from the piece and omit the other. It is very important that the writer writes in a way that the reader goes through the entire piece and retains as much about the piece as possible. It can only be done by writing objectively.