Say you're coding some kind of web application. Something where people can contribute content, e.g. a simple photo-sharing site.
How many good reasons can you think of to not go with an object-oriented database (e.g. db4o)?
An OODBMS is better if you only need to access your data through your objects. If your solution requires additional pathways to your data (e.g. ad-hoc queries, reporting, other applications that need data access but can't make use of your objects), then a traditional RDBMS system is better.
Note: OODBMSes have made a lot of improvement in this area.