The answer to your question is that it depends very much on what you hope to do in the future. If you want promotion of some sort, e.g. to teacher trainer or a director of studies post, or if you want to change school or country then it’s a good idea to do a certificate course. Even if your aim is to make this sort of move in a few years’ time, it is advisable to do it as soon as possible. Some posts require a certificate in TEFL and 2 years post-cert teaching.
If you are happy in your teaching situation, it is not essential to study further. If your reason for studying is purely to learn something new, you could just read some of the recommended books for certificate courses. However, if you decide to do a certificate, you will definitely learn something new or be reminded of something you haven’t done for a while. No matter how long you’ve been teaching, I’m sure you will learn something on the course.
I hope I’ve answered your question; maybe I’ve asked more questions than I have answered!