I have a 50-135mm with an "unreliable" SDM.
It sometimes works and sometimes doesn't. There is no pattern.
The most frequent problem is that it just refuses to start autofocusing after a lens change.
Of course, the contacts are clean!
Then, all of a sudden it starts to operate!
In my country, servicing this lens is not the easiest thing.
I do not see the reason why some members seem to ignore the need for a button (or menu item) to click and start operating the screw driven focusing!
What harm could that do?
Let us say your HSM stops working in the middle of a shooting session (remember this is a "star" lens, directed to higher end users). Would you call your favorite local Pentax technician to help?
I also doubt, by the way, replacing the motor with a similarly constructed one will make it reliable. The next generation motors may well be alright but I have the present generation lens
So, I repeat, what is wrong in allowing us to switch to the screw driven modality when a need arises?