I've had BHS Gold insurance for good number of years, despite being mainly an RS rider only. When I'm paying to ride i.e a lesson or hack with an instructor present their insurance covered me. But I also did a short term share elsewhere, plus the RS were happy for me to fetch a random beastie out of the field/stable and ride out with the mature liveries. (I'm 58.)
I felt it was only fair to cover myself. My choice, my ride, my risk. The horses I've shared/borrowed have been insured by their owners too. I just felt it was useful to be able to say I had my own cover for 3rd party liability, etc. I never claimed on it but it for about £60-70 a year you benefit from BHS advice/helplines etc too.