I didn't hire a wedding planner, but there were SO many details with my wedding, even though it was teeny, that I can see the advantages of having one. Still, I watch "Whose Wedding is it Anyway?" and I can see how good wedding planners can go horribly awry ;-)
But in the end, I think it's a good idea. Someone to oversee all those details. To help you create a theme, or a vision. You only get married once, which means by and large you have no idea what the heck you are doing, LOL, and should hire someone who does!
When my son and daughter-in-law married, they hired a wedding planner. She made life so much easier. If there was a problem, we deferred to her. She was high-energy, effecient, and really tactful, and at the rehearsal dinner, wedding, and reception, she seemed to be everywhere at once, making sure everything was running smooth and everyone was happy.
If a person is having a pretty big wedding, I can sure how a wedding planner would take a lot of the stress off.
I didn't hire a wedding planner but it was such a small wedding I didn't need one. If it is a large afair I would suggest to hire a planner just to keep the stress down.