The point is that by squinting one reduces the focal length so that what is on the slide one would see faintly if at all.  If the image on my microscope doesn't change if I remove the slide, I might think I'm not using it property or that it's broken.  But I don't know why people who use one professionally wouldn't realise this so I'll test it more.  As one person I spoke to pointed out, as children we didn't have access to microscopes in science lessons in school, or certainly not regularly, and one person I spoke to who I think said he was an engineeer said he hadn't learnt about microscopes since O levels.