If your only goal is memorizing the material, then sure. But usually it is a goal in of itself, to apply knowledge to novel problems and develop a solution. There are no solution manuals to unsolved problems. If you went your whole graduate education not working on the ability to approach problems you haven't seen, you will not do very well in research, and many aspects of professional life.
But sure, if you do a first pass without a book, spend sufficient time trying to figure it out, then a solution manual is helpful if you are not getting the feedback loop of a professor.
Well the feedback loop is much faster with a solution manual than a professor. I agree there is a line between leveraging the solution manual to teach yourself more quickly and just copying the solution. My quantum mechanics professor would give us the solutions to the homework before we were expected to turn it in so that we could learn from his explanations.
I agree that you have to a some point learn how to approach problems which you haven't seen before, but almost every field I have seen, the right way is to start by copying solutions of others until you understand it and then riffing from there. My father was a professor/researcher and he said you shouldn't start a problem unless you knew what your solution was and what you expected it to provide.
In fact much of the research work I have done is see if technique from field x will apply to field y after I have become an expert in field y. Or push to edge of field y and take the next logical step. But pushing to the edge of field y almost always requires working through the solutions of the people who have been there before rather than reinventing the wheel.
But sure, if you do a first pass without a book, spend sufficient time trying to figure it out, then a solution manual is helpful if you are not getting the feedback loop of a professor.