Here's a bug your test suite probably wouldn't catch.
A backend developer refactors the user model. The id field — an integer since forever — starts coming back as a string: "42" instead of 42. Every value is still "correct". Your assertion pm.expect(user.id).to.eql(42) fails, sure — but only on the one endpoint you asserted id on, not the other nine that return users. Meanwhile three client apps that did user.id + 1 are now computing "421".
That's structural drift, and it's what actually breaks API consumers: renamed fields, changed types, properties that quietly vanish. Field-by-field value assertions catch it patchily and by accident. Schema validation catches it systematically — and in Postman it costs about ten lines, because the ajv JSON-schema validator is built into the script sandbox.
The ten lines
In Scripts → Post-response on any request that returns a user:
