It matters because there was a failure and it could be in a technology or service you also use.
It matters because many users re-use credentials.
Scenario: You send a confidential email to a colleague, colleague has her lastfm compromised. Attacker scripts up logins against all common sites - including her Gmail account where you sent your confidential email. Script not only logs in and changes password, it also forwards to her friends and family all the emails containing some target phrases - including your confidential one.
anyone care to bet everyone with a linkedin account who works at a tech company just got targetted attacks? Hi, my name is Joe Hacker, and I work at <domain name>! Since you got my linkedin account, why not try that password at admin.<domain name>?
Anyone who uses the same password for their linkedin and company account earned their punishment for intentionally violating their company's information security.
Protip: Whenever you are evaluating a system and there is an unacceptable failure mode and your answer is "let it fail, then blame the user", go back and find a different answer.
It matters because many users re-use credentials.
Scenario: You send a confidential email to a colleague, colleague has her lastfm compromised. Attacker scripts up logins against all common sites - including her Gmail account where you sent your confidential email. Script not only logs in and changes password, it also forwards to her friends and family all the emails containing some target phrases - including your confidential one.