Notation formats attempt to describe what is written on a musical score.
MIDI attempts to describe what is actually played by instruments in real time.

Agreed, but because MIDI is a standard, albeit, devised for the latter purpose (i.e. to allow electronic instruments to communicate with each other) and because almost all notation processing software MIDI include import/export as one of their features, MIDI is a useful means of transferring musical scores between different notation software. I have been doing this with noteworthy for some years now and while a certain amount of "cleaning up" is necessary it is generally not too arduous.

Your best "human playing" MIDI for example is likely to be useless to anyone wanting to produce a score.

True, but that is not really the point here, unless you are trying to enter notation in real time and some notation processors do permit that.

MIDI produced by notation software does not have the little inaccuracies of timing that even the best human musicians produce. For that reason it is a useful way of transferring musical score information between different notation software.

I agree that MusicXML is aiming to become the standard for storing musical scores electronically, but as far as I can see it is long way from being universal. I suspecte the real breakthrough will be when one or both of the "big two" agree to use it as their default "save" format.