To paraphrase Tom Kyte from Oracle (Ask Tom): "Your company paid good money for Oracle and all of its features. Use them instead of wasting money reimplementing them poorly yourself."

This applies just as well to the open source world. You may not have paid money for PostgreSQL, but you did pay for it in that the time you spend maintaining it could be used for other things. Likewise, your time could be spent reinventing its features in your application or just using those features and spending your time building something better for your customers.