It clearly depends on the integrity of the coach. Every coach is different. I've seen very forceful parents try to win favor for their child and get it. I've also seen forceful parents come upon a coach of high integrity and it seems that parent makes no difference in the line up.
Parents also see things from a parental point of view...can't always see the big picture. Coaches may have a long range plan that the parents aren't privy to. There are always lots of things that parents don't know that are going on - even though they think they know. If a kid is playing and parents don't think they should be, there may be some underlying reason for it from a coaching perspective...
or maybe not. The coaches' integrity and knowledge of the game is all parents have to rely on. There are great coaches, good coaches, and not so good coaches. If you happen to have a great one (always the ones with the highest integrity) - consider yourself and your child very lucky - the same with a good coach. If you happen to have a not so good coach or one without integrity, then it is unfortunate for your child.
Overall, I think parents usually know in their gut if their coach has the integrity to do what is right for the team.
So the question becomes (for those with coaches that don't have the team's best interest at heart) "What do we do about it?"