Luckily I don’t have any accidents (and never had as far as I can remember), so it’s always on purpose. If I protect the bed or not depends highly on the temperature outside. Within the warmer months, I’d say most of the time is unprotected, as I use a fan over the day. Usually, the bed is dry when I go to bed.
In the colder months, I use protection most of the time so the mattress isn’t staying damp, which could lead to nasties.
Therefore I think using protection or not can't be answered with right or wrong as it is more about personal circumstances and preferences 😌.