Mainstream theory about learning to play the piano (and other instruments as well, I assume) is that it is necessary to use a metronome to learn to play correctly in rhythm. For the beginning pianist, it is taught that you should use the metronome to help you learn to keep a steady beat to and assure that your quarter notes, eighth notes, etc., are all in their proper place. For the intermediate or more advanced pianist, the metronome is used as a tool to gradually increase your speed, when attempting to play a fast piece, by moving the metronome speed up a notch at a time.
Both of these are terrible ideas. Here's why:
1. It is not necessary to use an external tool to learn to hear a steady beat. We have a beat within us (our heart and pulse) 24/7. Sometimes it is faster, sometimes slower, but always there. There is nothing more natural to humans than feeling a steady beat.
2. Regarding correct rhythm: it is necessary to train the student's ear to hear units of time, and then divisions of time, in order to be able to hear rhythmic patterns. I use an analogy of learning to chop a log without benefit of a measuring tape. If you want to cut the log in four equal pieces, you eyeball the whole log, find the midway point, chop there, and chop each of those pieces by finding the mid-way point as well, and so on, if you wanted eighths and sixteenths. If you want three equal pieces, you will notice is this appreciably more difficult, since you don't have the ability to cut in half. However, with practice it can be done. The same is true for hearing rhythm: you learn to hear a unit of time then divide it in half (or thirds). When you have this basic skill you can cut in more complex combinations of these. See my earlier posts titled Rhythm for a more complete explanation of this technique. Essentially all rhythms boil down to divisions of twos and threes; even music in 5/4 is heard as 2+3 or 3+2.
Using a metronome thinking it will help you get your rhythm correct is a tricky business. Let's say you set the metronome to the speed of your quarter note. If you are only playing quarter notes it should be easy to follow the metronome (however, for something that easy you wouldn't need the metronome, would you?). If you have a more complex rhythm with lots of eights and sixteenths and dotted rhythms, how could you be sure you have the correct divisions of the beat when you only hear the tick of the metronome on the quarter? You could be playing the eights or dotted eights or sixteenths incorrectly, but hearing the "tick" on the beat may lead you to believe you are correct. If you really wanted to be sure, you'd have to set the metronome to the speed of your eights, or even sixteenths, and in the latter case, make sure you hear 4 ticks go by when you have a quarter, or 8 ticks go by when you have a half note. As you can see, this would be terribly cumbersome, tedious, and prone to error. And the worst part is that you'd spend so much of your energy listening to the "ticks" you would never really hear the music. Of course, people who have learned this way would say that "counting" (saying "one-ee-and-uh" for sixteenth notes, for example) would guarantee that you'd have them correct. But, as I've stated in my previous posts, it is certainly possible to say these words out of rhythm as well, so there is no guarantee.
If you have a syncopated rhythm (strong or emphasized notes on the off beats) it would be really tough. And if you have cross-rhythms (two different divisions of the beat such as normal eighth notes in the right hand and triplets in the left hand) then of course you could not use the metronome to help you with that at all, since you could only set it for the beat, and it would be the two different divisions of the beat which is the challenge. My point here is that the metronome can really assure you success only with simple rhythms, where you don't really need the help, and cannot help you with complex rhythms, where you need the most help. (See my earlier post on Polyrhythms for an explanation of how I teach mastery of these.)
Using a metronome is not going to teach YOU to really hear rhythm, or to see rhythmic notation and immediately be able to play it. The metronome will always be external to you. Rhythmic understanding needs to be internalized. You will become dependent on the metronome and will most likely find it hard to wean yourself off of it. Some African and South American music has very complex rhythms, far more complex than Western Classical music, and I can assure you that the drummers in those cultures learn how to hear and play all those rhythms without ever having even heard of a metronome, much less having used one.
3. Using the metronome as a tool to play with increasing speed: Learning to play with dazzling speed and accuracy is not simply of matter of playing slowly and gradually increasing the speed. Think about it: if it were so, almost every pianist could do it, because virtually everyone starts out playing slowly. Playing a very fast arpeggio, for example, requires a different use of your arms, hands and fingers than playing it slowly. It must be more efficient, and cannot have any extraneous movements which would limit your speed. Learning how to play in that way is an advanced skill and can be taught by a talented teacher. (Whenever I teach a new technique, I teach it in such as way as to apply to both fast and slow playing.) If you aren't being trained for fast playing right from the start, you may never achieve that ability. Using the metronome will not work if you don't have the basic technique to play fast. You may keep notching the metronome up to faster speeds, but eventually you will reach the maximum speed of your ability and no amount of using the metronome will give you the ability to play faster if you haven't been practicing that skill all along.
To make matters worse, this practice would be very time-consuming, especially if it were a long piece. There are better uses of practice time than playing through the piece over and over at slightly increased speeds.
You can see that the metronome does not accomplish what people often think it does. But it also does harm. With the metronome ticking away, you surely cannot employ any nuances of timing, such as ritards (slowing down) or strettos (speeding up) and all the subtle flexibility of time that all music needs (but especially Romantic music such as Chopin, Brahms, etc.). Just like any other skill, you need to practice doing this to become good at it, and you can't when using a metronome. Playing frequently with a metronome can, and will, make you play more mechanically.
Furthermore, it is imperative that you hear the music you are playing without extraneous noises. You wouldn't want to practice with people talking in the background, so why play with the annoying "tick-tick-tick" of the metronome?
I've found that, with very few exceptions, students who were forced to play along with the metronome grow to hate it, and it is often one of the main reasons they quit the piano. There is no reason why the learning of rhythm has to be so painful. Rhythm is a natural part of our beings and our lives. It is up to the teacher to find ways to make the learning of it a joyful experience.
I don't own a metronome and haven't owned one for the last 45 years. I've taught hundreds of students to play the piano, many from the very beginning, and have never used a metronome. There are very few who didn't learn to have an excellent grasp of rhythm. (The ones that didn't were, in almost every case, the ones who had used a metronome excessively with previous teachers and couldn't, or wouldn't take the risk, to learn not rely on it, despite my help.) When a new student, who already owns a metronome, asks me what to do with it, I always have the same answer: "Burn it."