It depends on the person and the subject.
Just because one person can do well in an exam doesn't mean they are any better than someone who doesn't. It does depend on the subject as well though because, for example, I know for a fact my sister is excellent at looking after kids and anything to do with kids but she couldn't pass an exam in childcare. Some people a really good at creative writing but can't pass English exams. But then, for a subject like maths, if you can solve a maths problem anywhere then you can do it in an exam so, in that context, exams are the best way of testing people's capabilities.
I personally think that, for jobs such as childcare, construction, decoration etc. people shouldn't have to sit an exam or have a certificate to get the job because it's about personality and practical skills rather than knowledge. However, for jobs in the sciences, humanities etc. exams are an important proof of knowledge!