A good spotter can make you a better pilot.
The AMA Safety Code clearly states that a spotter must be used when appropriate. That is most of the time. I usually test fly my airplane designs when no one else is around–I don’t need to have a spotter in those cases.
The AMA See and Avoid Guidance document says to use a spotter to help keep your distance from full-size aircraft.
It takes a lot of practice to become a great spotter. You need to always be a step ahead of the pilot so you can tell him what he cannot see yet. A spotter is another set of eyes on your side.
Once on a windy day I let a model get so far away from me that I could not see which way it was heading very well. My spotter helped me get my model back, which I know I would not have been able to do on my own. Tip: use spotters with better eyesight than you!