I've had a Squeezebox for about 4 months now. I've updated the server one time in those 4 months, and another release has just been made, but I have not installed it yet. In any case, the server has had no problems, so there is not reason to upgrade if you don't want to.
As a side note, SlimServer is developed on an open source model, so builds are updated nightly. A lot of people who develop the server, or want to test it, will install these builds. However, it is not recommended for average users, because they are development releases and likely to have bugs. A lot of the traffic on Slim Devices' lists are people talking about problems with the development builds. This may be the reason why it seems like people are updating all the time.
So, for the Squeezebox, updates come every 2-3 months or so. I run the server on my Mac OS X box, and installs are painless. Just a couple of dialogs to click through. I can't speak for any other server software, as this is the only one I use.