Non functional requirements are just as important as functional ones. Yet not much thought is given to them (especially at the beginning when writing the specs for a new "system"). For instance everyone, obviously, wants a secure application (also security is a bool, it's either secure or not, there are no intermediary steps) so a new application will not be released if it's insecure no matter how many functional requirements are implemented.
The idea is to always keep in mind non functional requirements when designing a new "system". Some of the non functional requirements (like capacity/performance or security) can even have a direct impact over the functional requirements.