He most certainly uses Debian's packaging for any dependencies he may need.
Or do you think he also compiles stuff like Readline, IConv, Zlib, OpenSSL? Or stuff related to Ruby gems, like postgresql-server-8.4, in case he needs ruby-pg ?
On other platforms even installing GCC is a painful process, when on Debian/Ubuntu is as easy as:
sudo aptitude install build-essential
And why would he need to compile his own Ruby for stuff unrelated to developing the Ruby interpreter? Are you telling me that if he needs to do some quick processing on some machine he absolutely needs HEAD?