The role of a monitoring system is to reproduce sound without adding or taking anything away from the original input signal. Modern recording systems have a flat electronic frequency response. To accurately monitor what is recorded on the hard drive or tape machine, the monitoring system also has a flat response at the listening position.
Secondly, onto the often quoted "final mix translation" issue; one can observe that domestic and car audio systems are generally improving over time and having better, i.e. flatter, frequency response. As a general note, a good mix should sound good on any system. One exception to this rule is the X-Curve used in the movie industry. Movie theatre replay systems are installed in very large rooms (e.g. a movie theatre for 200-800 people) and the frequency response across the audience area is never flat. The Dubbing Stage must replicate this response so that the mix translates precisely to the Movie Theatre. Please note that the soundtracks for the release of movies on DVD's or Blu-Ray´s are re-mixed on flat response monitoring systems for reproduction in domestic environments.