Include guards vs #pragma once
#pragma once operates on the absolute filename of a file. Include guards work on the content of the file.

If you have multiple copies of the same library (maybe one library has some parts of another included in it - it happens...) #pragma once would have no hope of working right.

Add to that the fact that the Arduino IDE copies some files around the place when building and '#pragma once is pretty much screwed.

Include guards work regardless. So sensible people use include guards and shun #pragma once.