Thank you, thank you, thank you, Phil.
The question isn't so much about whether constraints are a good idea, it's more about understanding the psychology of developers who don't want to know that they're a good idea. It seems to me, that in the sphere of application development, it is too easy for bad database design to be implemented. But why?
I suspect I'm not the only one who has to deal with software developed by (usually highly intelligent) people who are NOT developers by trade, but who:
are very good at what they do in their particular profession of choice
enjoy dabbling in software development as a kind of hobby
can see where some quick wins could be gained by a "simple" bit of software
because they are intelligent and good at what they're trained to do, develop an arrogance that there's nothing they could possibly learn from database designers. "All that guff will just slow us down and cost a fortune" ...which is true in a very narrow sense.
What happens is that hobby software ends up in production, and is on the face of it, much better than what was there before (paper processes for example). Once it's in production though, removing it eventually becomes akin to cutting out a cancer. Things that worked OK for a few thousand rows eventually start to disintegrate, but only after hundreds or thousands of business dependencies have been built up around them.
I deal with databases from around 20 different software vendors. The seven most problematic (i.e. over 30%) followed the above path on the way to becoming commercial software. The sheer audacity and arrogance of some hobby developers ("I know the business, therefore I know every aspect of the software requirements") is a large part of their commercial success. As sales people selling to people in their own professions, their cases can be compelling. I honestly don't see a cure.
I was sitting in a meeting with a vendor once and we were attempting to discuss the manifold data integrity problems we were having because of the non-existence of constraints. The vendor simply closed the discussion down by saying "It's not the software's fault that the data entry staff are idiots." And that was that.
:-)One of the symptoms of an approaching nervous breakdown is the belief that one's work is terribly important.