[cesaref]

So that you can cope with someone messing up a deployment and wanting to roll it back?

Let's say you avoid column restrictions in your db design, and you deploy some dodgy code that doesn't fill in a column correctly, now you've got heaps of dubious data cluttering your database, and apps failing because some precondition isn't being met. That doesn't sound like a good compromise to avoid headaches when updating a schema, does it?

So as ever, it's about making the right decisions on a case by case basis, and there is no 'one size fits all' for stuff like this.