  Reusing ontologies in practice is still very challenging, especially when
multiple ontologies are (jointly) involved. Moreover, despite recent advances,
the realization of systematic ontology quality assurance remains a difficult
problem. In this work, the quality of thirty biomedical ontologies, and the
Computer Science Ontology are investigated, from the perspective of a practical
use case. Special scrutiny is given to cross-ontology references, which are
vital for combining ontologies. Diverse methods to detect potential issues are
proposed, including natural language processing and network analysis. Moreover,
several suggestions for improving ontologies and their quality assurance
processes are presented. It is argued that while the advancing automatic tools
for ontology quality assurance are crucial for ontology improvement, they will
not solve the problem entirely. It is ontology reuse that is the ultimate
method for continuously verifying and improving ontology quality, as well as
for guiding its future development. Specifically, multiple issues can be found
and fixed primarily through practical and diverse ontology reuse scenarios.
