Chris Date advocates an extended-relational model, and Informix implements it with DataBlades. But the operations are still relational, and data still exists in tables. This is a good approach for a particular class of common problems and should replace many of the existing simple relational databases, which do not need a full object model. You could use an extended-relational database for employee records with fingerprints or a photograph but not for a multimedia library with interactive objects or for a model of the stock market. A pure object database has methods, classes, and other things that characterize the object-oriented model in the database engine.
Objects are active.
Relational data is passive, and you need a host program to do something with it. They are designed to solve a different set of problems. To paraphrase Esther Dyson again, using a pure object database to store relational data is like keeping auto parts fully assembled into cars and disassembling the fleet when you need to count the screws you have in stock. One eventually asks whether this is the most efficient way to conduct an inventory. The argument is that having a standard language has made relational databases less expensive to build and much more portable across products and platforms.