True, but I'm sure that same music textbook had plenty of space for composers like Mozart, Schubert, etc. These composers really didn't advance music experimentally either (and I think Schubert was practically unknown while alive) but textbooks still include them in the history of music, because although they themselves did not advance music theory or performance practices, they influenced later composers (like Mozart influencing Beethoven). It is my belief that Rachmaninoff should be mentioned because he plays a very important role in the history of music. Although his music did not dissolve into intellectualism, an entire generation of pianists looked to his playing as influence. Notably his style and the highly sentimental nature of his music have provided an incredible amount of music, and not just to the realm of classical music. Rock and pop tunes frequently quote melodies from Rachmaninoff pieces. And surely the neo-romantic composers drew inspiration from Rachmaninoff - look at Lowell Liebermann's music, one of the most important composers alive today. The style is that of Liebermann, but he must have been influenced by the sheer power and "Rachmaninoff" style of composing and playing music.
Modern Music History textbooks have unfortunatly fallen into the trap of being "politically correct" - this is why you have composers like Amy Beach and Scott Joplin who have huge sections devoted to their music just because they were women or minorities. In fact, if Wagner was not practically indespensable to music history, he probably would have been left out completely. It is my belief that music should transcend race and gender, and should be judged on its own merit - composers like Rachmaninoff and Scriabin who are frequently ignored, should be placed above composers like Fanny Mendelsson where they belong.
Topic: Rachmaninoff haters