actually these modern CPUs are capable of somewhat overclocking (You can see from various examples like samsung is releasing their galaxy s as galaxy s plus overclocked to 1.4ghz) so no, it is not injurious to the health of the processor under a limit. Custom ROMs are OCed for various reasons like smooth user interface and yes mainly for playing 3D games, but you can control that with SetCPU, remember overclocking doesn't mean that you have to run your phone at the same overclocked speed all the time, it just gives you the facility of overclocking, you can underclock also and overheating is taken care by undervolting so it doesn't overheat and battery backup doesn't take a beating