I do consulting for a lot of different companies. The older ones who did not religiously use PK/FK constraints have databases that are nightmares to maintain. The data will start to rot surprisingly quickly, devs will react by writing weird code to compensate, and your life will not be fun.
I worked in different banks for the better part of a decade, lots of internal applications, each with their own DB.
They all had PK/FK/unique/check constraints, and they were still a clusterfuck to understand and maintain. Lots of outright stupid and dangerous code to work around the issues.
Still, if there were no constraints, it would have been even worse, as some 'intelligent' people temporarily disabled constraints to update data, resulting in data integrity hell.
