I would describe a certification/degree as proof that you have a basic underlying level of knowledge of the area you're going to work in. It's great for getting you in the door. In many jobs, it would be very difficult to take a high school graduate off the street and give him or her the extensive training he or she needs just to begin to learn their everyday job functions. All the same, a degree is in no way an indicator of acquired skill or experience.
However, if you get hired off the street with no knowledge, and after 5 years on the job you have learned everything about it, I would say you are just as competent as a person with a degree. The degree still helps though. If you quit a job, if you are competing for a new job with people who have a degree AND your level of experience, you will likely be passed over.
People do like college degrees, not only because of the specific knowledge you've gained, but simply because a person who has attained a higher level of education is more likely to be well-rounded and have an easier time picking up new skills.
Still, education can NEVER trump experience. Education in itself comes from those with experience teaching others.