I'm less likely to use triggers, but I'll say I pretty much always want proper foreign key relationships set up in the database. Unique and other constraints too. In principal I might agree with you that it's an application level concern, but being able to setup these kind of invariants and trust that they will be enforced even in the face of bugs in my code (and there will be bugs in my code) is just too powerful to let go of in the name of purity. I'd much rather let a bit of business logic creep between multiple layers that discover that I have a whole bunch of orphaned records and no sensible way to reconcile them.