I think a double, or even a triple, would be useful on some occasions having read Matt Page’s accounts of the Ironbike race in the Alps…
As has been said – what works best for one person won’t for others… It all depends on fitness (the biggie), terrain, bike weight, riding style… Personally, where I ride at trail centres, N Downs, Swinley and XC race courses I find 1×10 or 1×9 perfect for my blend of all those things. I ride past a lot of people on the uphill bits at such places for whom a 36t single ring would not be perfect. As I said, it all depends… This isn’t one of those questions that has one answer.
GB