Ontologies raise heterogeneity
I found this interesting quote by Pavel Shvaiko and Jerome Euzenat at http://www.dlib.org/dlib/december05/12inbrief.html "Ontology Matching"
However, in open or evolving systems, such as the web, different parties would, in general, adopt different ontologies. Thus, just using ontologies, like just using XML, does not reduce heterogeneity: it raises heterogeneity problems at a higher level.
I have viewed ontologies as rather subjective and at times arbitrary systems of organizing content matter. Therefore, this quote appealed to me. However, Shvaiko and Euzenat are also talking about 'matching ontologies' in their article as a solution to the above mentioned problem. This approach combines analytical strategies of different fields to combine the knowledge contained in several ontologies in order to get from it, what you need at a specific moment in time. Resources on ontology matching can be found at the Ontology Matching website.