We have recently observed people creating customer accounts automatically on Magento sites in bulk. Errors in their tools make it obvious that all the accounts they're creating are linked, such as identical corrupted data in signup fields, even though the registration HTTP requests are spread out over time, from a variety of IPs, and using different useragents.
We have never seen these accounts log in or return after the creation step.
So given someone's collecting a list of working customer account/passwords for a particular Magento instance, what purpose could these bulk accounts be for? Is there an attack or exploit class that this is a sign of / warning for?