First of all, politics is more than "Liberal" and "Conservative", as we also have Green politics, socialism, social-democracy, liberal-conservative, liberal-progressive, left-progressive, left-conservatism, and so on. (Since the US only has two parties, you may easily be ignorant of that). People who put either themselves or others into too much of a certain direction can be seen as "stupid". As McCain acknowledged, he hated leftist journalists, but acknowledged that he needed leftists to disagree with him. Some of the argumentation leftists came up with did make sense to him, even though he disagreed, and sometimes he had to admit the leftist were right about a few subjects all along. And the same goes for leftists something having to see that about certain subjects the rightists are right all along. Since the key to most problems lies in the middle ground, or when being blinded by your own vision, you may sometimes forget the price of your visions (any vision can both be wonderful as harmful), and then you need people to disagree with you so you can see if you didn't push things too far.
The "if you are not with me, you are my enemy" attitude, which I taste in U.S. politics more and more can only lead to either stagnation (best case scenario) or even civil war (worst case scenario). And if you have no opposition at all, you not only killed democracy (which is actually the last bad result, oddly enough), but you can also do whatever you want regardless of the consequences, and if you don't pay the price, others will, and with nobody to stop you, things can only get worse eventually, no matter how well-meant your intentions are. Every good action does have negative effects as well. Opposition can make you aware of those.