When compiling, it matters that your computer is stable.
A computer can be quite unstable and still run Quake 3, Unreal, etc. for hours on end.
Overclocking your computer does not necessarily make your computer unstable - however, it is very difficult to ensure that your computer is stable.
Remeber the Pentium III 1.13 GHz? It only demonstrably malfunctioned on Linux kernel compiles - the problem is that most other programs don't tax the system to the extent that compiling large programs does.