You have to understand that the practice of cryptography has always had a military basis; the commercial/private use is ancillary.
So, what's "a serious security mechanism?" Presume you're a military commander during active war, whose battle plans are intercepted by an opposing nation. What is the likelihood, given the opposing nation believes your plan will lead to their complete destruction, that they'll be able to break the security in time to execute a counter-operation? A serious security mechanism is anything that reduces that likelihood.