User.toString rturns the users full name. 

However a full name may not be unique and as such when used in log messages we get something that is less than helpful.

toString is generally only used for for debugging and logging aids and as such we should print the user's Id so we can actually identify the user. 
This also makes it easier to use in log statements as you can just use the code user  rather than user != null ? user.getId() : null  (which when coupled with logging that may not happen removes some code without having to utilize messageSuppliers or surround a one line block with if (Logger.isEnabled(Level.BLAH)) { ... }