I have had two previous PiP Reviews over the last few years. I always get called for the interview. Last year, I received my paperwork far earlier than I normally do .... Replied, and waited. Recently, I picked up the phone and unexpectedly a voice introduced themselves as a member of DWP Staff ...... No letter or warning.
Without going into major details, my application had been passed for authorisation to this member of staff without going to interview. There were a couple of interesting things mentioned during the chat ..... Importantly, they commented ..... "Let me just check .... Yes, here's your confirmation of diagnosis and physical limitations from your doctors. And you have no mental health issues".
With regard to this comment even though it says on the application to not bother sending letters from doctors, I had sent the reports. And the DWP individual added that ALL the files they were seeing had those type of letters. So, I have Fibromyalgia but it is secondary to an Autoimmune and severe Arthritis in my spine, neck, knees, and hips. My letters confirmed the limitations it causes to my life.
Up to you .... But if you are applying and can obtain a doctor or consultant's letter then I definitely would. I can't see that it would harm your application.
I got my PiP and additionally, it was given for ten years.