What I also found a bit offensive was the tediously PRIMITIVE works that were supposedly hand written by Brahms himself. Um... WRONG. I am familiar enough with the work of Brahms to know that the trite waltz we heard Spock play was nowhere near anything Brahms had written (one would have thought a little research and some cleverly written waltz by a modern composer with obvious elements from the style of Brahms could have been used). But again, that can be overlooked. James Daly did a terrific job with his role. So did Louise Sorel with hers. While a simplistic episode from a set and effects perspective, it was a refreshingly interesting story with a surprise kind of ending.
Remember, they only had a few days. The liner notes to the box set...
...say that Wilbur Hatch asked keyboardist Ivan Ditmars to compose the new waltz, and Ditmars got it done in three days. It may not pass muster for a classical audience, but I've always liked it. The waltz has a good bridge that's dialed way down in the episode, leaving only the simpler part for most viewers to notice.
I second your comment on the guest stars. Louise Sorel really hit the spot for me. And the big reveal, when Kirk finds the Raynas who were junked for parts, is like a really good TWILIGHT ZONE moment.