Some information for the discussion:
The following four use cases are provided by Eric Meyer.
It has been suggested that many subgrid use cases can be solved by display: contents. This demo shows in a simple way the types of things that aren't.
This use case was posted by Shaun Rashid to my Grid AMA site. See discussion and link to a CodePen.
Use cases from fantasai, gridded form and page layout.
Jen Simmons indicated she was working round subgrid on this example. She wanted to line up the two grids lined up vertically.
Searching Twitter for subgrid turns up some discussion from authors who are just getting to grips with grid layout, see this thread as an example.