It's useless in most circumunstances: it depends on what the attacker has access to. If the attacker only has the master password hashes the server uses to gate access to the encrypted database, then they would need to go through the full 200,000 iterations. But if they have the encrypted database then they can run 100,000 iterations and just try to decrypt the database (assuming this check is cheaper than the 100,000 iterations, which it likely is). And it's far more likely the attacker has the encrypted database (which can be pulled off of any machine which has been logged in, as well as the same database which contains the master password hash, if bitwarden's servers were to be breached) than the master password hash alone.