Streaming isn't buying. You're perpetually renting. Once you stop paying the monthly, you no longer have access to that movie you like.
I only buy DVD or BD copies of movies that I really enjoy. Usually timeless stories that can be enjoyed over and over again. Or sometimes brainless action movies with good production values.
As someone else pointed out, one of the benefits of physical media is that you get the full theatre sound experience, assuming you have the right a/v equipment. However, I've heard that streaming certain Netflix movies through Internet Explorer (Silverlight), you can also get multi-channel audio.
Another benefit to physical media is no buffering. Believe it or not, broadband internet still isn't widely available. So if you live out in the stick where your only choices are dial-up or satellite internet, loading a DVD/DB is much more desirable.