You technically can have plain staves if you go into engrave mode and drag out a frame and choose to populate it with blank staves from the little drop down selector at the top of the music frame box. That said, it wouldn’t have a clef assigned to it. So depending on the context, it may be more useful to use the music frame option in engraving mode rather than the assign and hide approach. I’d guess for some worksheets it will end up being a combination of both.