Readability score is a useful metric in its own right, is based on a well-respected mathematical formula that the military developed, and will not be replaced with something else. (If you end up with a block that you like better, you can always disable it).
There are benefits to both types - the readability score (and others we have planned) is generated automatically using tried and tested formulas, and does not require additional user interaction. A feedback tool can be more organic and potentially more relevant to a given community; however, it requires a great deal of user interaction directly with the tool before the statistics become useful on a broader scale.
What you describe is also a useful tool that could be added to our list of blocks. I am just thinking in terms of implementation now... it is likely more suited to use the ratings system or polling system, as that kind of block, rather than a necessarily dedicated block.