A very good question. My main issue is the durance of support. Ubuntu LTS for example will expire after 3 years and then upgrading the versions can become quite the hassle (last time I tried I ended up in a situation where several people told me a clean re-install would have been the better solution).
The same applies for other distros as well; take Centos (which runs on my Internet servers). Which even has issues on its own given that most stuff needs to be "ported" from RHEL. Linux isn't the answer for me. I have seriously looked at FreeBSD (due to the ZFS support) but that seems even worse wrt support, for example:
8.2 with a support lifespan of 2 years is pretty short when looking at Ubuntu LTS which can provide 3 years.
And well, to get to the point:
As well as my Office environment:
I can use the 2k3 environment for at least another 4 years with security updates. And it is not too unlikely that this cycle is going to be extended. Would I then consider upgrading the hardware (probably a good idea) I can then look into 2k8 (which by that time should be more affordable as well) and then also enjoy several years (at least 4 - 5+) of updates.
Office; I bought it cheap and I can use that for 3 whole years with full security updates as well.
"Free as in beer" sounds cool, but when you're trying to run your own (small) business you'll soon notice that "free" only goes so far. The moment I need to stop working and instead work on maintaining the software I use for my business its no longer free; then its costing me money.
Like it or not; but paying for an older product which gives me a longer support lifetime is much cheaper for me than not paying for a product while having to upgrade it much more often.