I think one learns to recognise the sort of compositions/scenes that will work in B&W, vs those where removing the colour info will kill them. However personally, there are many times when I want to look at both possibilities before deciding, even if in 80% of cases I go with the B&W.
That's why I don't understand the interest of a mono camera : it's not as though I'm exposing expensive sheet film and really need to visualise the scene in advance in B&W (and I'm not aware that ground glass provided that feature back in the day either). I press the button and decide later, since my card holds 600 raw images (and I'm not a "mitrailleur"). I guess it's conceptually possible that seeing the image in B&W on the LCD would make me notice some mono-specific compositional issue... but even then, I can arrange that via B&W jpeg if I want.