I think "almost never" would be more accurate here.

Mercator is only used because it's the projection you get when you don't apply any projection to longitude/latitude. It's a result of the map creator being lazy.

I could equally well say it's lazy to use just use Google Maps or some other convenient mapping service that uses Mercator projections at the world level when those services are not really optimized for displaying at that scale.

Many orthogonal, North-aligned projections will look fairly similar same when zoomed into a street at least a couple degrees latitude away from the poles. Ideally, you would always use a 3D projection of that section of the sphere (e.g. Google Earth not Google Maps) or a local two-point equidistant projection since these are always the most accurate. An orthogonal projection only suffices in common usage because there is so little need for maps near the poles.

In any case, I should have qualified my original comment with: "for a whole-earth map or a map showing distributions". Mercator is always a bad projection for a map showing distributions because it seriously misrepresents area, undermining the effort to accurate portray distribution.

You see a lot of Google Mercator because it's the native projection of Google Maps and pretty much all subsequent slippy map libraries and tilesets. It's a great projection when you want something that works easily with map tiles and is zoomable. But it's not a great choice for a whole-earth visualization.