IMO the main reason subject tests are not as important as before is that students applying to top 100 schools are taking more rigorous AP exams. I mentioned elsewhere issues of integrity due to recycled questions that seem not as bad on the October exam. There are also issues that similar types of questions are repeated. They were not designed for students studying as intensely for them as occurs now.
The topics on the science exams are very broad, reflecting honors high school classes 30-70 years ago, which covered topics in college classes in a watered down way. Some of the topics themselves reflect older curriculum rather than current textbooks.
While they are less important in admissions than before, they have some uses. They can be helpful for students who were not able to take AP or IB classes, but can still show off there knowledge by doing well on them. Also, many competitive prep schools continue to offer their own curriculum, which may be harder than AP classes. Combined BS/MD programs generally want close to 800 preferably in all math and science subject tests. Internationally, admissions is generally mostly on free answer subject tests. Many students in other countries take them, similarly to US students, as an extra on applications in universities in their own countries.