People have been able to successfully complete with the top Pros without a track of their own; however, its far easier to understand the effects of changes made to a car more quickly with your own track. In turn, the learning process can be accelerated.
Trying to become just a mid-pack contender, I struggle with identifying problems with the different cars I build. A microscope (which I now do own one) can certainly validate my polishing technique, both on the wheel bores and axles, much more so than a jeweler's loupe can. A seasoned builder may not need one after the process is learned.
In my case, I was burning through parts, not knowing if my axle polishing was as it should be, not knowing if my bore polishing was as it should be, etc. With a microscope, the feedback was instantaneous, with a clear "picture" of possible improvements. Its not right for everyone, but for me, I want to compete at a high level (not mid-pack or scout racing), and a microscope is a tool to help me do so. I need to eliminate every variance possible.