You can't really say this without the data. The placebo effect is quite strong. For example, pain relief from sham surgery can be just as effective as real surgery. As I said, there is nothing wrong with utilizing the placebo effect, but we don't have to cover it up in magic and fairy dust and teach it as doctrine.
It's only one more class as a part of a whole doctorate degree (Can't say the same for chiropractors), that, unless was taught in the way that we learn it, cannot really be applied/learned in any other capacity.
Learning OMM is not just about the manipulations, even though that's the "big thing" that people know. But, in actuality, teaches osteopathic physicians a whole approach to a patient. Even if you don't utilize the manipulations after school, it still teaches you to look for certain stigmata which could be indicative of underlying visceral organ pathology. 90% of OMM is just palpation skills to narrow down a long list of differential diagnoses. And yes, of course, you do need to go ahead and order Imaging and bloodwork, but imagine if you can shave off one 80$ test just because you "felt" something that pointed your differentials in another way.
At the end of the day, the placebo is nice, and perhaps you're right and all we are doing is making them feel good with placebo, but unless you actually learned OMM/Anatomy/Body Kinematics you wouldn't be able to do the proper thing to even incite such a placebo effect. Or worse, you could actually hurt the patient.
A lot of OMM is good even just for diagnostic reasons. Imagine if a patient has a certain pain and you're 90% sure it's just musculoskeletal, and you do something and the pain goes away. That's good, you just at least helped point your further treatment in the right direction and now you don't need to order an MRI/EMG thinking there was nerve damage.
So, I agree, OMM could all be just placebo, but, I don't see the harm in learning it, even if it didn't help the patient at all and it was all fake, it is just another tool in the huge tool box the Physician has to narrow down pathology.
And in some patients with horrific end stage liver disease, sometimes the "all natural" approach is all they can tolerate, and I'd rather be able to provide them with that then nothing at all, even if its just all placebo.