It's a useful ploy to argue for philosophical or political space for your views. How can you be shut out if it's all relative?
Once you have pretension to controlling the philosophical or political space, the idea is utterly toxic? How can you argue for your views to win if the other views--those you hate--are just as valid?
You left out the slogan that you "can't legislate morality." Of course you can try, and both the right and the left have tried for decades. Both have been left very frustrated and cynical, and typically resort to more cynical and draconian attempts as time go on. If the plebes would just do what we want!
The arguments have often been moral. They're often framed as utilitarian, but ultimately, for many, they're moral. Lakoff is at least honest at this, and older, utilitarian-based thinkers suck at Lakovian framing. I just saw an attempt perhaps 45 minutes ago on DU. It's amusing in some ways that more than a few of those who utterly oppose things they view as contrary to the establishment clause are closely allied with proposed policies rooted in religious arguments while dissenting from things like having a certain symbol that costs little and affects few on some forsaken bit of public land. They agree in morality and at that point view religion as external trappings; while the external trappings of those that they disagree with cause fury.
Lakoff's point is that the right frames things in moral terms. Therefore the left must, also. It's conveniently the framework of choice for "us vs. them" them thinkers on both sides. This is toxic to thinking. Moral arguments are fast arguments, easy arguments, often tribal arguments, but seldom rational arguments. In this, Lakoff is saying that (D) must appeal to the lowest common denominator in ways that avoid serious thought in order achieve virtuous and high-minded ends.