hmm,..good question,..and i bet every overclocker or normal pc user has his own opinion in the matter.
why do it?.....well,..if you're constantly annoyed with slow, slow..processing speed,application, games, etc....and you still haven't saved enough cash for that new computer that youve been dreaming about.( this is considering that youre gonna do it correctly and your mobo and ram can take the beating)
is it dangerous? ....yes! if done incorrectly,..it'll ruin your system,..fry your cpu. leaving you like a dead duck,..infront of a monitor that displays blank.
if you're planning to oc,..have sufficient,..in fact have tons of cooling aids/fans....
read you mobo manual and don't over do it