I would say that engineers would be terrible if they couldn't apply fundamentals from their field with their eyes shut and no reference material, but tbh a lawyer who couldn't memorise a large amount of cases and what not off the top of their head would probably be equally as useless.
But it's unreasonable and unnecessary to ask a lawyer to do that. First of all because far as the public citizen is concerned, a lawyer's/solicitor's job is to tell you what rights are available to you and what courses of actions are available to you, not necessarily where they come from. Just as if you were to go to an engineer for advice on a project you would expect them to explain to you why it would or wouldn't work, not the formulas behind it. You would assume they understood the formulas and knew how to apply them, the same way a lawyer would say "the law provides that..." or "there have been cases where..." without mentioning the specific case/law names because it's irrelevant to you. The lawyer/solicitor would know where they came from through having applied it in their studies or in previous cases (because you don't forget them easily once having done that) but ultimately, what difference does the case name make for you as long as you can trust the lawyer knows it or is able to find it? Every lawyer will always do their proper legal research before litigation or giving their clients a formal letter of advice, so having to know it from the top of their head is unnecessary. Furthermore, there are a limited amount of laws that will give definite answers to any case since cases are all subject to their own facts, which is why in training they are taught that everything has to be phrased as a "likely" or "unlikely" outcome, and to never get technical with clients. In that way, knowing a case and law is not enough in pretty much all cases if you can't prove that they should be applied. It's guaranteed that any half competent lawyer will know cases and what not from the top of their heads, but given that everything they do doesn't require it until after the initial meeting with the client it's completely unnecessary.
Secondly because unlike the laws of physics, laws do actually change - regularly too. This is why there is such an emphasis on being able to apply your knowledge of the legal system and your legal research skills. So what's the point of being able to rattle off laws and cases if you don't need it when communicating to clients and if you're going to have to do legal research in every single case you handle? Isn't it more important to know how to apply the laws that you will find (because seriously using legal databases are retard-proof once you figure it out), and more to the point being able to actually find the relevant laws?
Something else to consider is also how previous cases has been considered in previous cases. This is not something you learn in your studies, but something you come across when you do your legal research. So when it really does come down to it, the most important thing a lawyer needs to be able to do is research.