This may be more of a practical issue: for some scripts such as Korean,
representing every possible character and partial character could
require a very large amount of codespace. We only have the precomposed
characters now for compatibility with platforms that simply dont support
composition whatsoever (all too common still, sadly).
For example: do these both work under your mailreader?
NFC: Tiếng Việt
NFD: Tiếng Việt
(for me under mozilla mail the second one looks slightly different,
which means its not working perfectly)