Well some are better than others right. Riak/CouchDB are great when it comes to clustering/sharding. MongoDB has the best JSON support and is very friendly for developers. MySQL is proven, has the best tool support and most widely supported.
The fact is that just like one programming language doesn't satisfy all use cases. Neither does one database. So maybe PostgreSQL 'supporters' should stop trying to claim it is perfect for everyone.
Which RDBMS do you have in mind ?