Ok, my bad; it does work. What confused me is that I can't see subdivisions in edit mode. In the picture: a cube in edit mode with a subdivision modifier in simple mode with 4 subdivisions; subdivisions are not shown on the screen.
You can see it in wire frame mode or make it visible if you turn on "on cage", the currently greyed out button next to the three blue ones in the modifier.
Plus you'll need to turn off "optimal display".