If I had a dollar for every time I saw someone doing application-layer-only validation of data and end up being surprised that some data that wasn't supposed to be valid ended up in the database only to later blow up in their face, I'd be a fairly well-off man.

In practice, declaratively creating data constraints at schema-creation time has a much much higher success rate than trying to imperatively enforce data constraints at write time, in my experience. The missed cases and bugs always come back to haunt you.