Yes I do think that all riders/owners should have insurance. I don't own my own horse at the moment, however I am insured. It is vital in this day and age to ensure that you are protected. Also it is one less thing for drivers to bang on about in their quest to ban riders and horses from the roads!
Insurance, along with a road safety certificate may go a long way to improving our lot on the roads. Education of road users, that is riders AND drivers, in the safe use of Britains roads is essential.