It's one thing to deliver a message effectively, to persuade others that your ideas are correct and your political opponents' ideas are not. It's a marketplace of ideas where the best idea can dominate.
As soon as it becomes about destroying your opponent's credibility permanently, so that you're the one that will be listened to no matter how good or bad your ideas are, you start to erode your own credibility when people see what you're doing.
It's like destroying the competition instead of beating them. Once they're gone, your quality goes down and your prices go up. You profit, but the consumers (in the marketplace of politics, the populace in general) do not.