Some of you are being a little too technical...
Imagine an average consumer, who works and has two kids, want's a server at home to stream stuff around the house, and setup logins for each person in the household. They could go out, buy a cheap PC or netbook type device, download a Linux distro, install it, and then configure it with a static IP address, spend hours setting up samba and getting the user logins/shares to work.
Not everyone has the time or want's to mess about spending hours of their precious time configuring and managing a server. The average consumer just wants something they can plug it, quickly setup and job done.
Sky + is important to many people these days, but you don't see people asking for dual raid storage just incase one of the drives fail.
Apple know fine well who their market it, and so far, they're doing a great job at catering for them.