One of the reasons that some people have a problem pitching notes is that their ears are slightly "out of tune" with each other. Try listening to note with one ear and then the other - you might find that one way round the note seems sharper/flatter. The FIE technique can help with pitching in this case. The technique is also useful when you're singing a tight harmony with someone and want to block out other sounds (ie an audience member who is well out of key in choruses).

However I usually find that the "Hand cupped between mouth and ear" method is usually the best way of providing personal unaccompanied "Foldback". It is especially useful in packed singarounds where everyone is trying to pop different harmonies in (the sort with Johnny Collins, Jim Magean, Tom & Barara Brown, Dave & Annie etc.). It was this method, rather than FIE that Ewan McColl made famous - I always recall him sitting the wrong way round a chair resting his elbow on the back with his hand between mouth and ear.

Unfortunately both methods are very difficult when you're trying to play an instrument with two hands. Perhaps some musical accessory manufacturer will come out with a clip on device (worked by a pedal) to overcome this.