Is an expert system better than a human expert? Sometimes.
An expert system can out-reason humans – especially non-expert humans – in a particular domain. If you have a good knowledge base designed to reason well, it won’t fail.
If the knowledge base contains very deep knowledge resembling advanced expertise, the system will reason like an advanced expert and outperform many humans, including some experts.
Think about a physician who has a lot of expertise in human health. She might not easily be able to describe the test results that indicate Short-Chain Acyl-CoA Dehydrogenase Deficiency in a patient. An expert system with deep knowledge in this area will know the right numbers. It will never forget.
If the knowledge base covers a very wide domain, it can outperform humans, including some experts.
Think about Google Translate. It may not be able to outperform a native speaker of any language. But it can do reasonably well in over 100 languages! That covers a lot of the ‘language domain’.
Expert systems can cover wide domains too. Think about a system that covers a wide legal domain, including personal injury, loans & debts, consumer, consumer, real property, personal property, and insurance law. It would be hard to find a human legal expert who could perform well in all these areas.
Humans can think outside the rules
Expert systems have to follow the rules in their knowledge bases. If the input or description of the problem doesn’t fit the rules, the expert system won’t work. If the problem doesn’t match any of the rules in the knowledge base, the expert system won’t be able to offer a well-reasoned judgment or even a guess.
Human experts can think outside the rules and deal with complexities that computers can’t always handle effectively.
It also takes a lot of effort to program a knowledge base. No knowledge base = no expert system.
These questions are all wrong anyway
Asking who wins in a showdown between human experts and expert systems is a nice academic question. But we win when expert systems and humans work together.