I reckon they'd be more likely to leave the finder scope on and feel an unpleasant burning sensation on their neck as they peer through the eyepiece.
Seriously though, if there is even a pinhole in your filter you would be in trouble, and although Pete Lawrence mentioned in passing that you should check it before every use I think that should have been emphasised more. Everything except light bulb filaments should be completely black when you hold it up to the light.
Even then I don't see a big advantage over just projecting the sun onto a white screen, completely safely.
I was surprised they didn't recommend the projection method, but taking the finderscope off was a good idea, don't forget you could have younger kids wanting to take a look through the telescope and might try to take a peak through the finderscope.
I didn't see it as a problem having the amateurs there, remember we all started off that way and it's easy to get a bit arrogant over things you think people SHOULD know.
I remember every time PM covered the sun he'd repeat over and over and over about not looking at the sun directly, it is important because as we know it can blind you in an instant.