> Ok, fine. But I'm not sure how this helps if you have six different systems with six different definitions of a customer, and more importantly, different relationships between customers and other objects like orders or transactions or locations or communications.

If you have this problem, consider giving RDF a look - you can fairly easily use RDF based technologies to map the data in these systems onto a common model, some examples of tools that may be useful here is https://www.w3.org/TR/r2rml/ and https://github.com/ontop/ontop - you can also use JSON-LD to convert most JSON data to RDF. For more info ask in https://gitter.im/linkeddata/chat