I think theres so much emphasis put on the orchestration because orchestration is kinda difficult. And it really affects the way a composition sounds. Compare one of Mussorgsky's original pieces with one of his pieces orchestrated by Rimsky-Korsakov. Large difference isnt there? Want to have even more fun, compare both of those with one of the orchestrations by Shostakovich. Orchestration is just as important as harmony and melody. Maybe even more in some cases.
I've always held the opinion that a composer's work should be across all the genres, chamber music, operas, and large orchestral works. All say something different. I think a lot of composers may view the symphony as the ultimate medium because of the rich history it has. Who doesn't want to be compared or compete with Beethoven or Mahler? Not saying everyone does, but a lot of composers do.
And aleatoric music is an interesting thing. I love John Cage and Henry Cowell, but I personally look at strict aleatoric procedures like I look at strict serialism procedures, it turns the music into a procedure, a puzzle for the composer or the listener. That I do not, nor will I ever, agree with. A composer should write down what's going on in their head, not what some dice are telling them. In my opinion of course.