I'm in the Marydoll camp, I have rheumatoid arthritis and osteopenia. My rheumatologist told me about 8 year ago that she'd get my GP to prescribe VitD and Calcium (Adcal) to help with immune system and bone density. The meds she prescribes for RA lower the immune system but they give patients their lives back, so no choice or contest really.
My consultant rheumatologist said that in our area of the UK (north west England) random blood tests in February would show most of the population to be deficient in Vitamin D. She isn't a fan of supplements, keen on a good diet and proper drug treatments but so far as VitD and calcium go, she recommends to her patients.
I feel very fortunate to live in the UK with our brilliant health service. The meds I'm prescribed are very expensive but paid for the the NHS. In my 40's I struggled to walk up or downstairs, I was in significant pain. The rheumatology consultant prescribed the first line medication, within six weeks I was doing four mile walks again (first time in a year). By 62, I needed the next line meds. Here I am at 71 - still doing that four mile walk, more slowly but still doing int.