It's useful for navigation in the open ocean without satnav or even a chronometer, which is what it was designed for in the 1500s. Not for much else.
Is the use of Mercator in schools common, globally? Based on what I've read on the internet it's common in the US, but I have no idea about other countries. In Finland I think I only ever saw Robinson or Winkel-tripel type compromise projections. Mercator was maybe used as an example of how projections distort things.
I thought Mercator became popular due to online maps like Google using it. It's convenient for tiles because it's square.
I don't think I've ever seen a Mercator map of the world printed out, though. Is that seriously a thing? It looks completely ridiculous. Every poster I've seen has been a more rectangular projection like Robinson.
Google Maps doesn't use Mercator — it uses a 3D globe. If you zoom out you can see the whole globe and there doesn't seem to be any jump where the projection changes, or any distortion of country sizes.
Edit: I just noticed that Google Maps on Firefox and Chrome is indeed 3D, but on Safari it is 2D Mercator.
It preserves angles, which is what makes it useful in navigation. Mercator is bad at relative sizes for places far apart, but when you look at a small patch shapes are less distorted. For that reason, online maps use a version of Mercator.