Well, the reason for me is just that I need a server OS (yes, at home that is)
The reason for this is that my personal pc belongs to the company I work for.
This means I have the machine for my personal needs (I have another one for development)
, but from time to time I have to install software on my personal machine for testing purposes etc.
And it's that software that needs a server OS
Apart from that, I think 2003 is a really stable OS, very good performance, largely compatible with windows XP (all xp drivers should work on 2003), ... really a nice OS - even for a home pc