Is there a central guiding principal, or mission statement?
one that whilst perhaps not immediately achievable is useful and used?
eg W3C uses:
The W3C mission is to lead the World Wide Web to its full potential by
developing protocols and guidelines that ensure the long-term growth
of the Web. Below we discuss important aspects of this mission, all of
which further W3C's vision of One Web.
The social value of the Web is that it enables human communication,
commerce, and opportunities to share knowledge. One of W3C's primary
goals is to make these benefits available to all people, whatever
their hardware, software, network infrastructure, native language,
culture, geographical location, or physical or mental ability. 
The homepage of the ecmascript official site does not appear to
have any relevant content or links.
Though it does state that ecmascript is "the language of the web"
However the wiki does provide Goals and Requirements  but these are
only indirectly related.
In my opinion, a mission statement is not necessary for Harmony. More, mission statements can get in the way. Everyone has one these days, but you can smell the hypocrisy from a distance in notable cases. I won't name them, and I don't mean the W3C here: although it is subject to criticism for trying to reinvent the web as XML, among other mistakes, its mission is unassailable. But that's a problem, too. How many warm-fuzzy-generic-non-controversial mission statements do we need?
These are directly related, not indirectly related, to the odds of Harmony succeeding in ES6 and probably ES7 specs that move JS forward.
A mission statement is an abstraction. These goals and requirements are more concrete or even operational statements of intent, and of legitimate means to ends, both of which are critical to agree on in order for Harmony to obtain in Ecma TC39.
My two cents: we don't need too much highfalutin' posturing or mission statement crafting. We do need discussion, including on live technical topics on this list that may be suffering due to side- or back-channel communications, or possibly just everyone involved being really, really busy.