When I bought my plants I didn't quarantine them. I got them from the same store too. They are all fine. I found that plants that aren't potted tend to dissinagrate and fall apart though.
Snails are one reason, disease another. Did you know ich can be carried into your tank on plants? If you QT the plants in a tank with no fish, the ich will die since it has no host. That is just one example I can think of off the top of my head.
That is it in a nutshell. If the plant tanks at the LFS are occupied with plants, fish, and snails (they are at the LFS I use), then QT is cheap insurance that you have not brought something in that you do nor want.