Brendan, the default role was used used by the v1 feature for extracting  a color space name from an image file path.  If strict parsing was not enabled and no color space name was found, then the default color space is returned.  The default role was also used by some other API calls, such as getIndexForColorSpace (although that is not the case for OCIO v2).

Not sure if your question was for OCIO v1 or v2 (that is, master branch).  The documentation you linked to is for v1.  Note that the v2 documentation remains to be written (and we are looking for volunteers to help us with that!).

The new File Rules feature in v2 expanded the ability for users to assign color spaces based on file paths.  If you're interested in that topic, please check out PR #904.

A v2 config must specify a default color space, but this could either be done using the default role or a default file rule.  So if the config does not use file rules, then yes, the default role is required.

As Troy wrote, the scene linear role is required by ociocheck, which also verifies that any supplied roles have color spaces that exist.  However it does not require any other roles and I don't believe any others are required.

Doug