I want to add (had to leave to get somewhere...) that it is one thing to talk about metaphors, as in "oh, he must have meant....harmony" but in application this will not work - it is the actual things that are yin and yang that count.
A TCM practitioner cannot provide proper herbs and acupuncture on a metaphor of "harmony" - he needs to know what specifically is manifesting yin and what specifically is manifesting yang and how to guage which in and of itself is deficient or in a surplus and THEN figure out how to restore harmony.
If I understand Dan correctly - and I may be off base as we've still not met in real life! - in the martial arts application yin and yang similiarly is NOT a metaphor for an ideal state of harmony but actual things in their own right to be reckoned with.
It is absolutely not metaphorical, that's becoming more and more evident - even in Ueshiba's own writings.