I, like many Home Theater PC owners, simply don't have enough storage. Even though Terabyte hard drives are available (and somewhat affordable), I still don't have enough. I have a dedicated seperate drive for music, one for Recorded TV, another one for movies, another for photos, and another for High-Definition content... and that's just ONE of my Media Centers!
The physical limitations of the drive also makes organizing difficult. I could mix my high-definition movies with my normal Divx and X-vid movies, but I don't have a drive large enough to accomodate both. Sure, I could make a simple JBOD array to make them a logical letter, but this will be a never-ending process of adding more drives to my box until it is as noisy as a construction site.