depending on the size of your model, right? If your dataset is huge, your ___domain model app can shard the data and provide a nice entry point to tune performance without touching any of the n apps who are dependent on the data. I'm not sure i've ever seen a database that lets you plug in memcached to speed up certain queries.
The model may still become a bottleneck if objects are tightly coupled and you can't scale out the model in a horizontal way. The low level nature and conceptual simplicity of the relational model allows database vendors to pull some pretty amazing tricks with scalability and clustering.
People aren't building big database centric applications because they don't know any better (so I would hope). It all depends on the context. It shouldn't be the default because it can be inflexible.
