Cuz some people use a lot more space then you do. I use mine for movies and music and games.
So that is why I have 2x 640GB and 1x 500GB hard drives. It all depends on what people need
Likewise I value data security, (after numerous drive failures over the years with resulting loss of data). I chose to cut my potential capacity in half and run mirror drives, and I still have about two to three times more capacity than I have ever used.
I keep my hard drives very clean...nothing extra at all.
I run 2 x 160GB Western Digital drives not in any RAID set up. I also have a 250GB Western Digital external for back up. I have a 500GB Western Digital Black coming this next week I am buying a High Def Video Camera for better quality home movies.
Primary Drive: This is the operating system and a few small programs...only about half full.
Secondary Drive: This has all games and some programs. This is also the primary storage for music, pictures and videos. About 1/3 full.
External Drive: This is just storage...music, videos and pictures only. About 1/4 full.
I'm using about 400GB and I have 2 750GB set up in RAID 1, so I'd say that it reflects my storage needs pretty well, given that my storage usage is constantly growing.