Its a good point you make. A lot of the stuff you learn seems to be only relevant to Aircraft engineers and weather forecasters.
I am just doing the American IR and the theory paper is completely different. It asks practical down to earth questions that will save your life rather than the CAAs engineering based questions.
None of the questions require a fundamental knowledge of the workings of a rate gyro or an INS system.
Some examples are questions like 'If the instruments show this (insert picture) where are you. Then they give you 3 answers only one of which can be right. In the JAR exams such a question would give 4 answers all next to each other .
The weather questions are more along the line of 'which document will tell you if there are thunderstorms on your route?' rather than 'when does the craichin affect hong kong and which direction does it blow in?'
As to what do you remember afterwards the answer would be very little. I remember asking two airline pilots on a flight deck visit if the speed of sound increased or decreased with altitude (at the time I did not know) , they did not know the answer but they still flew the plane safely and professionally.
I enjoyed the ATPLs but if they had been more practical and geared around safety rather than interpreting difficult english the money would have been better spent.