I can see how that would be true; though I believe you would have to master the former before being able to do the latter.
I don't think you have to really master it, but getting the basics right, sure. After all, if you draw your own schematic, you're going to be the only one reading it. It doesn't necessarily have to be professional quality.
That's only a portion of the wiring on a 26 year old Heil, on it's side in an attic but still as big as a damn basement freezer. There are no less than five relays on it, a couple being added later for whatever purpose. Apparently on this model they had separate xfmers, relays and everything for both heating and cooling. I couldn't wrap my head around it in the brief time I was up there.
Another contractor had so mangled the job of putting in a heat pump over this past vicious summer that the H/O gave up and got by on a window unit. Now the blower will not come on after the furnace fires (thank God it did fire on emergency heat).
I took one quick look at that mess and grabbed L1 off the primary side of the heating xfmr and put it to a low motor speed, verified common and let 'er rip long enough to make sure it was enough air to avoid tripping on limit. Continuos blower operation while d/c is on and heat will cycle via stat to get him by until we can replace everything.
Ironically, I had just received my RSES Residential Troubleshooting book which seems to have a very detailed section on reading schematics.
the picture above is why the good lord invented side cutters.......chop it out and re-wire it...its pretty basic stuff.....
it was working.... played with it.... now its broke.... whats the going hourly rate for HVAC repair