I never thought about this before, but I was wondering why they chose runlevel 2. Every other distro and OS I've used default to runlevel 3 (with the exception of AIX which also defaults to runlevel 2).
The Debian distribution (and hence Ubuntu, which is derived from it) does not define any differences between runlevels 2-5 as a matter of policy. It is up to the local system administrator to make use of runlevels as they see fit.
Since there is no difference between runlevels 2-5, a default runlevel 2 was chosen.
Most distributions imho default to runlevel 5 (RedHat, Fedora, SLES, OpenSuSE, Ubuntu) - meaning login with runnung X11 and GUI. You have to specify otherwise, if you do not want a GUI.
But the definition of runlevels does not seem to be fixed.
I rember Solaris used runlevel 5 for poweroff, 0 for halt (without power off).