The overclock depends on whether the actual chip is capable. Some are, some aren't (known as the chip or bin lottery). You don't have to overclock no. Some people enjoy it, some people get extra performance from a processor. The reason for aftermarket cpu cooling isn't just for overclocking, it's about cooling better (which usually means quieter as well). With a good quality aftermarket cpu cooler, it would easily be possible for someone to not only overclock their cpu but oc it and have it running quieter than a stock clocked cpu on stock cooling.
Planetside isn't the only game that would benefit from it, bf4 and others that are cpu intensive would see benefits. A moderate overclock (nothing extreme) won't really shorten the life of a processor provided it's cooled properly. Yes, if it was overclocked to the max producing temps into the 70's (c) or more and kept that way 24/7 it would probably shorten the life of the cpu to a few years rather than 6 or 7 years. Most people upgrade well before then anyway. A smart overclock (running 60c or under while gaming) could get them from 3.5ghz to say 4.2 or 4.4ghz stable and steady for years with no issues.