1) Medical school is certainly a trade school, or if it isn't I don't know what is. You are learning to do a job and the quality of your education can be judged on how well you perform that job. That's a trade school. Its certainly not the dispassionate pursuit of knowledge for its own sake, its a very practical professional education.
2) Its not that you don't need to manage pregnant patients as a non-ob, or psych patients as a surgeon, its that you need to know how to manage them within the limits of your profession, and not as a member of a different profession. When you're on a rotation for a profession you learn a lot of tips that are outside of your scope of practice, at the expense of a knowledge base you need FOR you practice. That kind of knowledge is best learned from members of YOUR profession practicing YOUR profession. Doing a rotation in surgery to understand how to manage Pediatrics patients who have had surgery is like a chef working as a farmer to better understand how to cook food. Its not that I can't imagine a situation where the education would help a little, but it certainly seems like it would be a lot simpler and more direct for the chef to just take some more cooking courses.
Do surgeons manage diabetics? Absolutely, they write sliding scale insulin and insulin drips all the time. Do they optimize a patients A1C as an outpatient, and therefore need to know family practice algorithims and diabetic teaching? Never, and I challenge you to find a surgeon who still remembers how three years into residency. Do psychiatrists manage pregnant patients? Absolutely, that's why they need to know exactly how all their antipsychotics affect the baby. Do they ever manage a patient in active labor, and therefore need to know all the details of stations, the expected delivery time with and without an epidural, and how to catch a baby? Absolutey not, it would be a slam dunk for the malpractice lawyers if they did. Do Pediatricians need to be able to recognize a surgical abdomen? Absolutely, happens all the time. Does it in any way change their management if they know what happens in the OR after they call the surgical team to take care of the surgical abdomen? No, not ever. What you learn on the rotations in medical school is how to mange other people's patients in a way that would be malpractice if you ever actually attempted it without getting that other team involved.
3) Once again, the guys who made this system up weren't interested in a broad base of knowledge or in intellectual naval gazing, they designed a very practical, nonsense free trade school education. They just happened to design it to produce old fashioned, WWI era GPs. That's why we have the rotations we have. After medical school and one year of residency a GP could set up his own office and do open abdominal surgery, closed reductions, amputations, manage deliveries, compound drugs, do the town's autopsies, and run all his own labs in the back of the office. Thats why we have such an emphasis on Surgery, Ob/Gyn, pharm, path, and even biochem. It also explains the courses that you don't need: you can have a medical school without a shelf or a rotation in radiology (then sciece fiction) or ER (then triage in the biggest cities) even though these days those are probably the most universal specialties. When medical school staff opine that 'this is not just a trade school' they're ignoring our history, and they're doing it because they are too lazy and cheap to adapt our trade school to the modern realities of our trade.