It depends on what you´re playn´. If you play lot of chords, pop music and all (easy music) I don´t think it´s needed but if you train techique and are working on your speed and acuraccy, than it´s the only thing that will help you to reach your goals.

If find it almost important as guitar ´cause I´m working a lot on speed and acuracy and you can clearly see when you progress. I thinks it´s a VERY important thing - METRONOME.