George Washington stated, "let there be no change [in Constitutional powers] by usurpation; for though this, in one instance, may be the instrument of good, it is the customary weapon by which free governments are destroyed."
Washington well understood that often the liberties of the people are destroyed not by those trying to do evil, but by those try to do good. In other words, in the hope of achieving a desired policy end, politicians have often skirted the law, ignored precedent and tradition, and otherwise undermined constitutional limitations. When this happens, the law is corrupted, and liberties of the people are no longer safe.
Simply put, the power to do good, is the power to do evil. Once constitutional limitations are shattered in the pursuit of a "good policy", those same limitations become irrelevant to stop "bad" policies. This is a most common path to the subversion and ruin of free peoples. Thus, we must stand firm to protect the Constitution - even in the face of the good.