One of the basic idea of Semantic Web is to allow user to reuse an existing ontology if it meets our needs. Alternatively in the worst case we should be able to use a part of it to fulfill our requirements.
At the same time if every user of the web starts to build their own ontology then there would be no common language and shared understanding about anything. There would be no interoperability of any kind between the two agents using those ontologies. There would be no global processing possible either. The exchange of message will not take place and different machines cannot interpret the messages either.
Thus without ontology reuse the very basic idea of Semantic Web is void. Reusing ontology to an extent is even more important than reusing a URI.
Until Next Time.....