Some coaches have a knack for developing relationships with parents that enable frank discussions about player performance and playing time. This requires real openess and trust, but it can happen. As soon as the coach gets defensive or the parent becomes insistent, the relationship breaks down. Its a risky situation, but worthwhile if all parties can accept each other's opinions. If a parent must speak to the coach, do it privately and with dignity.
Some parents go so far as to write letters to the editor or confer with administrative officials.
Some just make comments from the stands. But the effect is the same. The coach's desire is diminished and the team suffers. It is pretty demoralizing to try and do your best and in return people insult you and undermine your efforts. Most of the coaches in the world are volunteers.