David Hockney's 'theories'

After learning David Hockney's theories about the artists using mirrors to trace their subjects reflection left me with mixed feelings about it. The theory makes sense as a whole. Hockney made points about how there was a sudden change in the artwork of the 14th century artists. The figures in the painting were more anatomically correct as well as clothing and perspective. I particularly liked his point of how everyone was left handed in the paintings and how the subjects looked somewhat unnatural until they mirrored the artwork. But I feel a little sceptical about this theory as well. It seems like the process of being in the dark with only the light of the reflection to trace seems a little bit too tedious. But if an artist wanted those proportions bad enough they would find a way to do it.
I don't think the artists that used this 'technique' are considered cheaters or being untalented frauds. It is the same as using a grid or something similar. The artists used it to get the correct proportions. They still had the talent the paint an almost life like work of art.