Just because the camera operator doesn't complain about seeing the boom mic in the viewfinder does not necessarily mean that the mic will not appear later on when the scene is viewed on a full sized monitor or projection screen.

Viewfinders are small and contrasty; dark windscreens can easily blend in to a dark background on the set.

When I worked in Hollywood, we were taught to always wrap a strip of white camera tape (cloth tape) around the tip of the (dark) windscreen in order to make it highly visible to the camera operator.

Better to see the mic during setup rather than be embarrassed during dailies!