Why would anyone use that ? Best practices require that every user has a random (probablistically) unique salt. Just use bcrypt, or other password hash functions.

Nah, user logins/emails often won't be long enough or random enough. See https://security.stackexchange.com/a/8024/28881 and https://stackoverflow.com/a/5565071/2521092

Would it be an okay practice to prepend the username as extra salt, still using the randomly generated salt?

Why not just use a longer salt? The username is only going to reduce randomness. Moreover, I don't buy the presumed advantage: nobody is really parsing that message to mean someone else could have the same password.

Be careful about how you merge the username and the random salt.