I think the difference is in plurality. Setting goals for a quarter (or month, or year) to me is more of a team thing/focus/discussion rather than an individual's estimation of what they can do. And I completely agree with you that stretching a sprint into a quarter makes no sense if it is only for estimating one person's work.

Given that so much of what we do as developers typically depends on more than just ourself in an effort, I agree with what I think tadfisher is expressing; realistic delivery planning can be achieved by focusing on "teams and goals for the next few months" and cannot be by trying to answer "how long will <insert feature never done before> take?"