Wedding Planners can be many things, one of which is completely worth it on your day. They should be running the entire day, making sure you are not bothered with all kinds of problems that can arise. The best planners have gone over all of your wishes with you, and have built into the time line wiggle room for all of the eventualities that may occur.
That said, some run good control over the day, and some not so much. A good manager is hard to find, and that is basically what a planner does, they manage all of the aspects of the day, so that it runs more or less smoothly, your not bothered by the hiccups of whatever may come.
Weddings that do not hire planners run usually, unless they are very small, in a disorganized manner. Using relatives is a mistake, they should be enjoying the day, and lets face it, how many of us have fantastic managers in our families.