Prolifieration of URI schemes works against interoperability
This is a misunderstood URI scheme, where the misunderstandings result in apparent inconsistencies.
Some applications or protocols may constrain allowable URI schemes, which appears to go against the idea of universality.
Some applications or protocols may constrain the form of allowable URI: e.g. no relative form or fragment identifiers allowed.
Some XML applications, notable XML namespaces, adopt the idea of a base URI that references a collection of resources, and sub-URIs (path-children or fragment identifiers) that refer to members of the collection. This combination of base URI and local name is referred to as a QName.