You don't need a piece of paper to learn great techniques. Most hackers just learn what they need as they go so that they can diversify and learn a little about everything.
Even just knowing the basics can turn you in to the type of engineer who can take a step back and notice that something might be missing. Perhaps there is a security flaw that he/she spotted. That is then the time to dig and learn more to properly patch the security hole.
But if you spent all your time studying for some security exam that you may never fully need, you may then fail to have the basic fundamental domain knowledge to build products that your users want to use. Read How important is Domain knowledge vs. Technical knowledge? for more information on the distinction.
Additionally, there may be other areas where you may not have enough basic knowledge to realize that you have a problem. By learning a little bit about everything, your abilities to recognize when you need to explore a topic further will be greatly enhanced.
If you know absolutely nothing about security, then you may not have the sense to know that you should Never Store Passwords In a Database (In Plain Text), for example. So balancing your knowledge is important.