I have used several high quality (ie $1k+) spotting scopes, and yet I have a Barska 20-60x. Why? Well I don't shoot much further than 100 yards usually, and often times my rifle scopes have enough magnification to resolve holes in paper that far. When shooting irons or with a lesser scope, I have found that 60x will resolve bullet holes in a low quality optic like the Barska, out to about 100-150 yards.
Higher quality optics are nearly essential for longer range shooting and spotting. This works especially well if you are shooting with a partner. You also need a good, steady tripod to mount the spotting scope on.
The biggest problem, even with high quality optics, at long ranges is mirage. But some spotters use this to their advantage and can see the vapor trail of a shot. I personally cannot do this, but I know shooters who can, especially in the right conditions.