That is hard to do when they are in the broom closet still is the original, now very yellow plastic wrapping........
Or when it's in the aeroplane, so you can't have access to it for study before flying because somebody is flying said aeroplane (the standard US model).
The better UK flying schools get it right in my opinion - a shelf in the ops room with all of the official POHs on the shelf and accessible to everybody to study there, with photocopies of the most critical pages kept on the aeroplane in case somebody needs them when away from home base.
The syndicates I've been involved with in recent years have made sure that everybody has their own personal copy, but the original is kept locked somewhere safe as a "master" - many ways to do it.
But as for mandating it in the aeroplane in case it might be useful when away from home base - where do you stop? Tool set? Lifejackets? Spare spark plugs? All things I'll sometimes carry depending upon the trip - but I don't think any of them should be mandated.
Although, just
maybe, the checklist should be on board?
G