a) The composers put it on the score. Therefore the conductors must honour the intention of the composer whether they personally agree with it or not.
b) The exposition repeats serves the purpose of introducing the themes to the audience that would subsequently be developed in the "development section" of the piece. Therefore the repeats are there to ensure that the audience are completely familiar with the theme so that they would be able to recognised the elements being developed. If the development section is an example of harmonic virtuosity from the composer point of view then only by recognising the themes would the audience be able to fully appreciate the development section.
However I respectively disagree with him and believe that the exposition repeat should be skipped
a) Composers aren't god. The score is not sacred. Ultimately musicians are servants of the public not composers who are long dead. A performance is always a combination of the heart and soul of the composer and the performer and it is never exclusively 100% from one or the other. The conductor and performers should perform the work to maximised the enjoyment of the audience. If that means having flexible differing interpretations of the work that the conductor believes is better than what is on the score than so be it. They should have free reign to how much they want to adhere to the score and the audience is the judge whether there interpretation is correct or not. Now from my personal opinion, i generally prefer historically informed performance with period instruments that is more in line of the original spirit of the composers intention however I recognised that there are exceptions where I enjoyed work from more flexible interpretations of works. I don't think the HIP rendition of works should be applied to every aspect of performance even if I prefer it more often than not.
However I believe that b) deserves the most attention as it goes into the heart of the underlying "spirit of sonata form" that I consider the most important aspect of concert music.
If this was back in the 18th and early 19th century. I would completely agree with Robert Greenberg proposition that the repeats are essential so that the audience would be able to recognised the themes that undergo motivic development. After all in the 18th century there was no such thing as a "canon" in music. People create music with the expectation that it would be performed once or at best a few times and then the public move on to new composition with the old composition be forgotten. The idea that a work is significantly important enough to be repeatedly perform until the end of time to preserve it status as a culturally and historically significant work was really only something that started when Felix Mendelssohn became a conductor (he was the person who is most responsible for developing a "western canon" of music). Hence when the work is composed with the expectation that the audience is only going to hear it once, the audience must be able to completely absorbed the theme prior to hearing it be developed.
However we now live in the 21st century where not only repeat performances of culturally significant works. We now have the recorded medium. In the past to hear music you need musicians to perform it and hence every music performance was a special event. Nowadays we just need to click a mouse or press a button to hear music. Repeated listen to a work of music has never been more easier.
So the statement of "How do we get the audience to fully grasp the theme of the music so they can fully appreciate the theme being developed?" has a simple answer.
Simply relisten to the piece of music. Replay the CD/track etc. We no longer have to buy a ticket to hear a work multiple times. Hell the only thing it cost to listen to a piece of music is a hard drive space and time. It only takes a few listen to a classical era work to recognise what is a theme and what is being developed.
It's no surprised that the exposition repeat became less common as the existence of canon became more established. Composers wrote music with the expectation that there will be repeat performance and hence there was no need to repeat an entire section of work for audience to understand the work as they recognised that people would listen to it multiple times and not just for a single hearing.
The exposition repeat is an anachronistic imperfect solution to a problem of getting an audience to comprehend the work out of a single listen. It's a solution to a problem that no longer exists.
I believe that the exposition repeat should be abandoned because it goes against the spirit of sonata form. Sonata form is about having a structure of music follow a dramatic arc of a story. The exposition introduces the characters or themes, the development section has the emotional climax where the characters interact or the themes being developed and the recapitulation has the characters are changed from that interaction or the themes be resolved to be in the same key.
Does it make sense to introduce the character and then reintroduce the characters in the exactly the same way? When we listen to an opera do they introduce Isolde and then introduce Tristan and then go back to reintroducing Isolde and then reintroducing Tristan. It makes no sense.
Also my values of judging music by the principle of unified diversity which is the heart of what sonata form represents and the heart of what makes sonata form a great structure to based music with. Repeating an exposition note by note makes the piece less in line with the principle of unified diversity as the piece is more repetitive and hence less diverse. It also makes the piece less unified and more abrupt. After all you have theme 1 in one key and the piece modulates to theme 2 in a different key. Without the repeat the development section would modulate the piece back to the original key of theme 1. However with the exposition repeat the piece of music would automatically jumped back to the key of theme 1 with no transitions. Now since traditional sonata form structure has theme 2 be in the dominant key, jumping from the dominant straight to the tonic sounds relatively natural but it would sound more natural having a transition section leading back to the original key then a sudden jump.
In summary the exposition repeat goes against the spirit of sonata form and the spirit of "unified diversity" and it only exist as a imperfect solution to solve a problem that no longer exists. Exposition repeat should be abandoned and is my only major problem of historically informed performance.