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why is that bad practice?
Sometimes, you only have one database available.
Also, if a malicious hacker (are there un-malicious hackers? well, nevermind) knows your database prefix, it is much easier for him/her to exploit SQL injection vulnerabilities. If he/she does not know it, he/she has to guess it in order to e.g. use the SQL injection vulnerability to add him/her-self to the webmasters group