My experience is different and here is why:
(1) Coaches that demand "respect" by the players and parents have no issues with whatever side the parents are on.
(2) Coaches that are prone to disrespectful behavior tolerate parents that show the same qualities. The "come on Ref!!!" shouts are intensified with the parents joining in the call they disagreed with. Moreover, coaches that tend to argue calls tend to simply have no concept of the Laws relating to fouls (mostly, "handling" (or "handball")) and view every single foul through their rose-colored glasses, whether its trifling, careless, reckless or excessive ... everything is reckless or excessive. This lack of knowledge infects the parents who are all in earshot and their support of the stupid-ass coach magnifies the sideline discord. Depending on the age of the kids, the discord then infects the players.
(3) Controlling the sidelines is not the job of the coach. This is the "Team Managers" job, which is why I make all the team managers sit on the parents sideline ... even though they like to sneak over to the coaches bench. When I have a parent out of line, I address the parent's behavior with a verbal warning and make sure the "team manager" knows that both the parent and the manager will be watching from the parking lot if the behavior does not change. The team managers are generally very quick to jump up and address the problem. At half, I inform the coach as well.
(4) When parents are all on the same side, there is less frequent disrespectful or inflammatory comments at each other. Its amazing the courage displayed by parents when separated by 75 yards. Put them on the same side-line and "Mr. I'll Kick Your Ass" becomes just a parent. Its kinda like the anonymity of the Internet Forums we see here.
I tend to see far less "parent" problems with SCDSL and Cal South tournaments, like State and National Cup (that separates the sidelines) and more problems with CSL and Presidio that let's parents sit on opposite sides. In addition, its much easier to manage substitutions and ascertain parent v. coach disrespect when they are separate.