Rather than estimate range with a spotting scope, I prefer to use a laser rangefinder, which is a much more accurate tool, for that application. It is also much more compact, and much quicker to range with, than having to set up, and dial in a spotting scope.
I agree a rangefinder is more accurate. I also like to know how to use my reticle as well as rangefinders can malfunction, batteries die etc. I guess its no different than being dependant on a GPS without knowing how to use a map and compass. I have also shot in competitions as well where a rangefinder was not allowed. To each his own though.