Shortly after I bought my Weber kettle, I wanted to cook some stuff indirectly - so I went to Home Depot (they have the biggest selection of Weber accessories in my area). They didn't have the baskets in stock, but they did have the rails so I bought a set of rails.
I figured I would want to get a set of baskets later on, but the rails have proven very versatile. They can do things that the baskets can't do - such as you can adjust the size of the charcoal area easily... i.e. have a big bunch of coals on only one side, or you can turn them around and block off the sides and have a strip of coals running down the middle for doing burgers or the like.
The one time I see baskets making more sense than rails is if you are using the propane assist on a Weber Performer - you then need to have the baskets over the propane igniter to light them before sliding them over to the sides where they will cook things.