From Emacs 23.1 NEWS:
*** The Emacs character set is now a superset of Unicode. (It has about four times the code space, which should be plenty).
And more details later on:
*** In multibyte buffers and strings, characters are represented by UTF-8 byte sequences. The character code space is now 0x0..0x3FFFFF with no gap; code points 0x0..0x10FFFF are Unicode characters of the same code points, while code points 0x3FFF80..0x3FFFFF are raw 8-bit bytes.
According to Wikipedia, the BMP of the UCS has 65536 characters, the latest version of Unicode contains more than 107000 characters, and the UCS has more than one million code points. 0x3FFFFF is more than four millions.
What problems could be solved or how otherwise it is beneficial to have internal character set that is a superset of Unicode?