I suggest you take a look at MacOS X and WindowsXP and that will answer your question. Both come with a top-notch Web browser, email client, media player, text editor, image viewer, basic movie editor and Apple also provide an organiser and jukebox (iTunes).
RISC OS is supplied with none of these things.
I agree a media player is not actually part of the OS, but it is now commonly expected to be supplied *with* it. As Apple and Microsoft, both OS developers, supply such applications free with their OSes, then so logic suggests that that is what ROL (and now Castle) should be doing. Nothing to do with the principles of what constitutes part of an OS, but what the simple commercial reality is in the rest of the consumer computing industry.