Well, for one thing, I love living dangerously.
Actually, it's mainly because unstable has PHP 4.3.x (4.3.2+rc3-6 right now), which brings a small collection of niceties like
file_get_contents().
"unstable" is more stable than the name suggests, apart from occasional crashes of mozilla, various X and gnome components, etc. which I don't use anyway. Spectacular b0rkage of ssh and pam did happen a few months ago... but I take special care to upgrade such key packages, i.e. mark the package as "on hold" in dselect, monitor bugs.debian.org for about a week before taking the plunge.