From RDFaWiki

Re-using Vocabulary Terms in RDF Vocabularies

It is considered a best practice to re-use vocabulary terms from other RDF vocabularies. This practice is not only possible in RDFa, but highly encouraged.

Example

A common example of a basic set of terms that is re-used widely is the Dublin Core Vocabulary, which defines a set of common terms that are used across most creative works. For example the Media, Audio, MusicOntology, and Video vocabularies re-use many of the Dublin Core terms.

Reasoning

Typically, concepts are shared between vocabularies. For example, the concept of a "title" is shared across all creative works such as music, movies, television, books, sculptures and theater. It would be wasteful to re-define "title" in each vocabulary used to describe the same concept. Not only would it be wasteful, but it places a burden on the authors of RDFa documents to remember all of the vocabulary terms. The easier it is for RDFa authors to remember vocabulary terms, the higher the chance is that they will use those terms more often than the ones that they cannot remember easily.