I believe that at the bottom end blood tests are helpful for sure.
Even the NHS accept that a threshold of 250pmol/L is where bone protection can be reasonably assumed for the majority.
Personally I favour prof Studd's threshold of 300pmol/L, but the point is acceptance that there is a minimum threshold for health benefits, regardless of where you believe that lies.
Not everyone has symptoms, my mum had none, and when I was in chemical menopause prior to IVF, nor did I. But I absolutely don't want osteoporosis, which both my parents have.
Also if someone is taking a reasonable dose by whatever route, if they are getting no benefit from it, then a test is very worthwhile, rather than than telling them they must be imagining their ongoing symptoms or suggesting antidepressants instead, when they may simply not be absorbing it very well, or at all.
But I don't think that someone who feels well and is really stable on their treatment should have their dose arbitrarily reduced based on a single "high" reading, as this may have been significantly different had they tested at a different time of day.