Except that "I don't disagree" actually does suggest something different than "I agree"... can't you sense the hesitance, the "... entirely" or the ".. but" often implied by the former, as opposed to the straightforward support of the latter?

Or, to say it a different way, consider that there are more states than "agree" and "disagree"... for example: "I'm not sure". So "not disagree" leaves "agree" OR "not sure", which is different from "agree".

We now return to our scheduled thread topic.