Actually, they are good also for helping challenge your thinking. We need people that think differently than we do and know things we don't know. That's how we learn. It's difficult in a political context because of the bile it often comes soaked in, but if you can overlook that, there are things to learn.
I think it's important to distinguish libs from leftists. I've learned a ton from liberals. Leftists, not so much. Liberals are in a bad spot these days. They have had a home in the Dem Party since the 1960's and it's hard to leave home. If you do, where do you go? There's no where to go except maybe to declare yourself a left-leaning Independent. So, most of them stay in the Dem Party, but as the party has increasingly embraced leftism, they have to go along to some degree and try to defend things they aren't entirely comfortable with that those revolutionaries to their left are pushing.
I would love to have dinner with journalist Ezra Klein, just listen to him talk about politics, and ask him some questions. I wouldn't agree with a word he says, but he says his words so well, and is very skilled as an apologist for the Left.
At the end of the day, leftist or lib, they will challenge you, and while they are rarely wholly convincing, they help you understand the parameters of the ideological battle, help you to refine your thinking, or, as is often the case, convince you further as to the folly and logical holes in what they believe.
So, be kind and thank a leftie today.