Actually not a bad idea. You could also hash the salt and use it everywhere instead of the user's ID - anonymize the user from their own data. (If you used the password hash, you would just have to remember to update it everywhere on password changes - and of course use a hash of the hash so you didn't give away which hash was used.)

For instance, let's say I was UserID 123 which had a FK to a table of bookmarks or history and normally it would be easy to link that user to personable data such as Cancer, Job searches, Pr0n, etc. Now instead, you have a lot of these bookmarks pointing to a hash that was used in the initial user login and not directly linked in the database.

Typically in highly sensitive databases you hash out a new "ID" entirely and reference that. Then you provide a different service and database entirely that correlates two different identities when you need identification. This is similar to how PCI requirements for credit cards store the actual numbers elsewhere and use a token against the system for consumption.