Some ear training teachers and programs suggest that knowledge of music theory is not necessary when training to play piano by ear. However, if the musician is to fully understand what he or she is playing and why, some music theory must be learned. For some, music theory is considered dull, monotonous and difficult to comprehend. It can take several weeks to learn music theory in its entirety, and learning the mechanics of chord structure and progression involved can discourage beginning learners.
Another disadvantage of ear training is there are too many methods by which one must choose to learn. If a student wants to learn to play piano by ear, she's going to need some kind of instruction, whether it be from a private music teacher or from an ear training program purchased online. One might not know if they're a good fit for a particular method until it's too late, when they have spent hours learning and realize they don't understand the technique or that the technique was flawed. Contrast this with learning to play sheet music, where notes and counting are constant, and there are definite right and wrong ways to play a piece of music.
Some students who begin playing the piano by ear find it difficult to transition to reading sheet music, should the need or desire arise. Part of this issue is due to the fact that they've relied solely upon ear training and chord memorization and can't make the connection between what they're playing on the piano and the notes they're reading on the sheet music.
One of the fundamentals of ear training rests with a theory known as perfect pitch. If a musician has perfect pitch, he can identify a tone in music and be able to translate that to what should be played on the piano. However, if one is not born with this ability, it must be learned, and it can be one of the most discouraging aspects of ear training. If perfect pitch is not mastered, all is not lost, but it certainly makes ear training a slow endeavor. In this case, learning by reading sheet music can help reinforce the notes a student is playing and help him identify the pitch simultaneously.