Graham - if you only needed the respirator to filter out say 5% (rather than the vast majority) of the contaminants there would be a good case for not requiring face fit testing.

These things used to be given a "Nominal Protection Factor", but it was found that most users didn't get anything like the NPF for reasons including poor face fit, with that being often aligned in part with stubble - to the extent that for some it was found that they could pass the face fit test at the beginning of a shift but fail it if they did not shave half way through the shift.

So, the rules were changed and part of the normal expectation is that face fit testing should be part of the regime except when using respiratory protective equipment where this is not an issue, e.g. helmets with forced air to provide positive presssure within the helmet.