There doesn't seem to be a general consensus amongst pianists and teachers about the effectiveness of the metronome.While almost every single teacher will invariably agree that the metronome is paramount in the early years and they encourage students to practice with it, many students seem to do fine with admitting they got away without every really using it, and the same goes for concert artists: The pianist Mischa Dichter says he uses it "about twice a year, which I need to borrow from my kids' room". The teacher-guru Graham Fitch admits as well he hardly touches it. By stark contrast, the pianist Ruth Slyencznska in her book "Music at Your Fingertips", recommends an obsessive use of the metronome - from the first minute a new piece is being learned, to "Six metronome speeds of every piece you intend to play on the day of a concert - Nocturne or Etude". At summer festivals I have overhead famous artist-faculty practicing with the metronome. The pianist David Dubal loves to tell his students that the "Metronome is the only teacher you'll ever need", reminding students that Chopin's metronome never left his piano.
First of all, a distinction needs to be made between beginning and more advanced students. In my experience, it is very trying to get a beginning or early intermediate student to play a piece beginning to end with the metronome. A few bars may go okay, but when a particularly rhythmical or technically challenging passage appears, they will stop or it will go haywire. While it may be possible to get them to do it, I've found usually the struggle and time is not worth it, and can ultimately lead to discouraging attitude towards practicing. (I know I never did this at that level). Far more effective is to encourage some preliminary drills with the metronome. I've had success with the following:
1.) Have them clap with the metronome as you call out note values. First, start with quarter notes. One clap per note. Without warning, I'll switch to whole notes. Then eighth notes. Then half notes, then triplets, and so on. Once this is mastered it can be switched to playing as the students plays notes, or chords, etc.
2.) Have ghe student clap so exactly in sync with the metronome, that the sound of the clap blocks out the tick - completely. All you see it the light coming on. Yes, this IS possible, and it's VERY fun. It usually takes a few seconds for the student to catch on and they will eventually drift off, but it engrains a strong sense of pulse.
3.) Have them take a particularly knotty rhythmic passage and have them hum it with the metronome. This takes away the physical challenges of dealing with the instrument and frees up the mind to concentrate on rhythm only.
After a while of the type of work, the metronome can seem much less intimidating and it is easier to apply towards repertoire. If the student continues piano study, as their overall technique and understanding of rhythm continues, it becomes more feasible to play larger sections or whole pieces with the metronome. I tell my students that the metronome is liked braces: It is this ugly, painful contraction that is only temporary, to smooth out the rough edges, and will ultimately leave their playing straight and smooth, and through it they will earn the right to play with an ultimately non-metronomic freedom and breath.
For the more advanced student or the professional who already has develop a sense of pulse, the purpose of the metronome becomes much more abstract.
When I was thirteen I began studies with a serious teacher who opened up to me what I've found the metronome's ultimate purpose in my life has served: A way to organize your practicing and keep it focused. Much of the piano literature has passages that require many hundreds (if not thousands), of repetitions over weeks and months. This repetitive work is almost unimaginable to those in other fields, and it requires almost superhuman patience and discipline. Personally, the metronome is how I've managed to complete these repetitions in a constructive way. To sit down and say to yourself "I am going to repeat this passage twenty times", is a recipe for self-torture and mental (or physical..) wanderlust. But to play a passage starting with the metronome at 60 and working up incrementally to 120, suddenly the perception changes from one of repetition to progress. Like a journey on a train, your eyes and ears suddenly become attuned to the subtle change in the musical and physical landscape of what you are playing at its existence in many different tempi. It's a slow-to-fast journey aided by a device that has a sense of subtly and perfection more than any human can have.
It's worth noting that the above has actually very little to do with rhythm, (the aspect of which the metronome is most commonly associated with..) and almost everything to do with focus. With the right mindset, the compulsive and annoying tick becomes a gentle and encouraging practice coach - telling us to be patient, that all great things are worth waiting for, and that our time will come.
It should be noted that ultimately performances need to be free and non-metronomic. Where I part views with Madame Slencynska is that in the ultimate stages of learning a piece there should probably be less - rather than more - metronome work. Certainly - if you always train yourself to play with a metronome your final product is going to reflect that. The very beauty of most classical music lies in the mysterious "In-betweeness" of rhythmic exactitude, those split-second breathes and pauses and breaking of the rules that are the moments we live for.
But until we get there, I think aptly of Tennessee William's wisdom: "Time is short, and it doesn't return again. It is slipping away while I write this and while you read it, and the monosyllable of the clock is loss, loss, loss, unless you devote your heart to it's opposition". The monosyllabe of the metronome is "Patience, progress, stick with me...you'll get this.."