... isn't a DBA paid to do his job? ...

Police are paid to do their job, too - does that mean we shouldn't complain when people rob banks? Developers who write code which accesses a database share a responsibility with their DBA to implement well performing, scalable, and supportable data models and access strategies. Frameworks such as RoR which do not provide datamodelers the flexibility to design a model the way they'd like to are by their nature limited. Sure, it many cases it's possible to work around these limitations but that's no reason not to point them out. It is certainly one of the reasons it's not caught on more broadly in enterprise environments where database performance is often paramount to rapid development.