Marking certainly helps establish control when a newbie wouldn't otherwise have it. Mechanically, it doesnt help much, but its a nice placebo. Think of it like.... that unhealthy comfort food we all have.
It also helps to have 1 almost dead things get ripped away on 1 target than having 3 barely damaged things running around on 3 targets.
.....Marking would also be better if people followed them pre-30. I have yet to have a group of level synced players follow a mark order in anything below Haukke. That gets really fun when people decide to burn down cap'n before his goons and then suddenly: dogs. newbie healers have fun with that one