This is a question that I hear more often than not. I came across this great article that really hits the nail on the head as far as the why factor is concerned. I couldn't have said it any different, so check it out here.
Now that being said, I couldn't agree more with the article, BUT there is also the issue of finding the RIGHT wedding planner. A wedding planner WILL free up your time, negotiate the best prices, let you relax and enjoy your special day, but you must have a great working relationship with your planner. I would say that most of the brides Liz and I have worked with in the past have become somewhat close to us - maybe even friends... We get cards and updates, e-mails and photos of new babies, etc. Bottom line is they need to TRUST us, and feel comfortable with us taking care of the details and carry out the vision that they have created for their day.
So, when selecting a planner, and you really should if you can somehow manage it into your budget, make sure you select the right planner, and not just the one that may be the least expensive, or have the most experience, or "seems like" the logical choice because your best friend hired them. They have to really "get you" and what you want for your day.
99% of the time if you hire a planner with great referrals from friends and family, most likely they will be great with you too. But if you are stressing out before your wedding because your not sure the wedding planner has the details under control or isn't executing your ideas correctly, re-evaluate why you hired them and communicate with your planner, or even change planners. A hassle? yes. Will you lose your deposit? probably, Is it worth it - definitely.
You only get to do this once...Why hire a planner? Because you NEED one, and they are their to HELP you.