When we start removing music history & theory courses and replacing them with more "popular" choices we are doing our future musicians a disservice.I'm probably preaching to the choir, but I have noticed a slow decline in the emphasis on music theory and history for young musicians. Other teachers have commented that class times for these subjects have been cut and there are threats to cut them even more. This is not a good trend, so I'm speaking out.
Performers of today (and tomorrow) need to be well versed in a broad variety of styles, even if they plan to focus on only one era or on only contemporary music. The audience of today is vastly different than that of even 20 years ago, and as such, they listen differently. For an example of changing musical tastes, listen to a recording of Bernstein's Mahler and then to Tilson-Thomas or Boulez; the interpretations are very different. Part of the difference is due to an individual conductor’s style, but part of it is that newer conductors are approaching the same music in a more modern way. I believe performers as well as conductors need to understand the music in its historical context as well as how it will be perceived by the audiences of today.
Understanding the theory behind the music is important for the best possible performance of any common practice period piece. While the composers may have used theory subconsciously rather than mechanically when composing the music, the performers need to have a mechanical sense of the organization of a piece - the way it flows - in order to understand how to effectively perform it. Music theory gives a performer the tools for understanding the bones of a piece and for interpreting the subtext of the music.
History of music goes hand in hand with any theory understanding. If a musician is to perform a piece of Haydn, they should understand the style of the music surrounding the period in which Haydn wrote his music. Haydn's music is very different in style than Schubert and thus requires a different approach. Even if the performer doesn't choose to play the music in period style, approaching the music requires an understanding of the framework for the original piece before any variation will sound true.
Learning the theory and history behind common practice pieces can also influence how performers approach modern pieces. If a piece has never been performed, or the performer is unfamiliar with the composer's work, there is nothing on which to establish what the composer intended other than what is written in the music. With historical composers there is a vast array of articles on how to interpret the music, how to play phrases and what constitutes a phrase in music. With new music, there is none of this background information.
Does this mean a performer of new music doesn't need to study theory or history? No. Schoenberg said there is no composer who doesn't rely heavily on the past in order to write something new (paraphrased). A solid understanding of what styles influenced the modern composer can provide invaluable information when approaching a new work. Modern composers can be influenced by the entire scope of music history (from plainchant to pitch class sets), a performer interested in new music needs to be conversant with the scope of music history, at least to some degree.
For example, recently a group of composers where chatting about rhythms they like in their new pieces - 6+4+6+6 or 3+3+2+3. These poly-rhythmic pieces are not necessarily new, but are very different than what you might find in a piece by Haydn or Mozart. An understanding of Stravinsky, Copland and Bernstein would help understand how to approach poly-rhythms.
In terms of harmonic language, some of these same composers like counterpoint, to which the rules Bach applied are still relevant. Other composers may choose more atonal forms of harmonic language, therefore studying Schoenberg becomes important for grasping how to phrase atonal lines. Rich melodic lines might be similar to Tchaikovsky or Beethoven, yet harmonized more like Debussy or Scriabin. Jazz idioms might be in play, so understanding the difference between Miles Davis and Thelonious Monk could help.
Music, like all languages, is a living form of expression, constantly evolving and changing. To be fluent musically, students need to understand the grammar (theory) and common usage (history) as a basis for creating new and exciting performances.