What they all said. While all elite riders use cleats, because they add a little more power to your pedal stroke, there are many many experienced, veteran, long-distance riders who don't, for various reasons. I commuted without cleats for many years. Everyone blathered on about how I should have them so I tried, and to tell you the truth I still don't think the added power is particularly noticeable. But I quickly got used to the ease of being attached and not being able to slide off, it's relaxing in a different way, believe it or not. So now I use them all the time, and feel very unsafe without, like I'm about to pop off my pedals any moment!

Once you want to try them you'll need a while to engrain the muscle memory, and once you do it's second nature. But there certainly are advantages to cycling without too, like wearing regular shoes.