The purpose of the @container:@set functionality (AFAIU) is to ensure that the output is consistent in shape. Thus if there can ever be multiple values, the structure is always an array.

There are two situations in which this functionality could be desirable but is currently not possible:

1. @type As it's a keyword, we can only alias it (e.g. as type) but not define it to have @container:@set functionality. Meaning that there's a gotcha waiting to happen for ontologies that require or use multiple classes for a single resource instance. See playground
2. @context Less useful, but @context will also compact to a single string/object when there might be multiple contexts. See playground

@context modifying itself seems particularly strange, but the inconsistency for @type seems solvable if the restrictions in its definition were loosened?