Currently, a POM-packaged artifact with a classifier cannot be imported - there is an explicit check for that in DefaultModelValidator. I couldn't find anything in Maven documentation that would mention/explain this limitation. According to Robert Scholte this might be "historical".
Example use case: a BOM (bill of materials) plugin could generate a BOM POM and attach it (with a classifier) to the project. However, currently there's no way to import the generated BOM because of the restriction.