Why server enforced integrity is relevant.
There are tons of reasons why placing integrity right next to the data (along with security and lots of other stuff) is important. This article just tickled another one in my head.
Maybe, must maybe, someone will change your application. It can be very easy - especially with "open standards" (common protocols). This very simple article spells out at a very very high level things you need to think about.
Just go to google.com and type in SQL Injection sometime. Read some of the articles there - think about what might happen.