Amusing though this anecdote may be, it gives a false impression. I've found that composers have an excellent ear for their own music, but they're not always organized in how they write it down. A composer's clear and practical instructions are essential for a piece to sound as it should.

Some composers misjudge balance, because they don't always remember which instruments are louder than others: for example, a flute solo in the low register is hard to hear if it's accompanied by a full brass section. Concerto accompaniments are often written too heavily; two excellent violin concertos by living composers spring to mind. The last time I conducted both pieces, I drastically altered many of the dynamic markings in the score (such as changing mezzo forte to piano), cut out some of the doublings in the winds (not using two instruments on the same note when one will do), reduced the numbers of string players in some spots, and occasionally used mutes. The librarians transferred my markings into the orchestral parts. Both composers came to the rehearsals and were amazed to find the balance was exactly right. I decided not to tell them why.