One of the many reasons to put good constrains on fields and use referential integrity! If you don't let the database enforce data validity you are gonna get fucked at some point!

source: every single place I've worked at that poo-poos referential integrity has a database that is full of bullshit that "the application code" never cleaned up

Always use referential integrity. The people who are against it almost always are against it for superstitious reasons (eg: "it makes things slow" or "only one codebase calls it so the code can enforce the integrity"). All it takes is exactly one bug in the application code to corrupt the whole damn thing. And that bug will happen over the lifetime of the product regardless of how "good" or "awesome" the programmers think they are....

... I'll get off my soapbox now!