How many non-embedded non-POSIX systems do you know? Windows also guarantees CHAR_BIT == 8 and since most software is first written for Windows or POSIX there is plenty of software that assumes that CHAR_BIT == 8. That means that anything that will want to run general software needs to also ensure CHAR_BIT == 8 - not to mention all the algorithms and data formats designed around you being able to efficiently access octets. The only platforms that can get away with CHAR_BIT != 8 are precisely those that have software specially written for them, i.e. embedded systems.