It alienates you from all sides.
There is a cost to listening to both sides of an issue: it makes each side think you’re a flaming idiot for not seeing that the other is wrong on its face. Whichever one you ultimately conclude has the stronger point will be less likely to accept you.
On top of that, it identifies you a risk factor. When you’re emotionally dedicated to a cause, you’re in. You’re one of them. You can be trusted. The same can’t be said when everyone knows you are perpetually one random factoid or rogue discovery from flipping sides.
No, really. It makes you look gross.
Double-checking claims about how many people of color are killed by white cops or how much women get paid relative to men jeopardizes your reputation as a compassionate human being.
If you uncover that a politician voted against a multi-billion-dollar hurricane relief bill because a) he thought billions were going toward things that weren’t relief and/or to agencies that have misspent or hoarded previous funds, and b) senators aren’t allowed to vote for/against individual elements of a bill, only the whole thing, you’re hosed. It doesn’t matter if you disagree with that politician and feel it’s totally appropriate for a relief bill to include funds for preparing against the next disaster. It doesn’t matter if you point out that he’s stuck in the no-win situation of either acting like an ass or voting in favor of what he and his constituents feel is corruption. The nuance is wasted. You’re already a shill for him, or a sycophant, or an apologist. Or you’re an idiot for believing those were his real reasons for voting no.
Approaching divisive issues rationally is not the optimal approach to affecting change.
It’s more sensible to strike a balance between learning about an issue and being an effective leader. As I pointed out in the opening paragraphs, you sabotage people’s trust in you if you thoroughly investigate the problem from every angle before committing to a course of action. The price you pay for making sure you know exactly what to do is that you may then find you’ve rendered yourself unable to garner enough support to do it.
It’s (possibly) selfish.
If your end goal is not to affect change but rather to maximize your self-edification, open-mindedness and critical thinking are wonderful tools…at helping you be selfish. An end goal of just making sure you know as many things as possible, and that all of them are true, doesn’t help anyone but you.
It doesn’t tell you who’s right. It only tells you who it’s most logical to believe.
Critical thinking isn’t a perfect approach to settling the truth of a matter. Research is. The only truly reliable way to tell who holds the more accurate stance on a controversial issue is to already know the answer yourself and check their conclusions against it. Of course, this is pretty much always going to be somewhere between impractical and impossible, so we fall back on critical thinking as a second-best-but-actually-workable approach. Always remember that it’s a fallback plan.
Critical thinking isn’t necessarily open-minded.
Meet Gene. Gene has a high school diploma but never went to college. Gene has worked for five years as a flower arranger at a shop in a small city in Vermont. Gene has some great ideas on how the U.S. should handle Middle East relations. Would you like to hear them?
Of course not! As far as you can tell, Gene has no expertise on the subject. There is no rational reason to expect to hear something insightful. The principles of open-mindedness say, “Yes, listen,” but rational thinking says to consider your source and advises you not to bother.
Eventually, you still have to pick a side.
Although it does require you to be forever receptive to the possibility that you'll need to revise your beliefs, being open-minded doesn’t absolve you from forming opinions or from becoming convinced that one viewpoint has distinctly more merit than the rest. Neither open-mindedness nor critical thinking encourage you to remain undecided forever, irrespective of what new truths subsequently come to light. In fact, if you’re going to do that, critical thinking actually becomes useless.
“Decide for yourself” is a rhetorical trick.
It’s counterintuitive, but if someone presents you with both sides of an argument (hers and an opposing one), then plays to your sense of critical thinking and asks you to decide for yourself which one to believe, you’re more likely to be getting shammed than if she simply argued her own position straight up. Like a bush league southpaw who can only throw strikes if he stays down in the minors, a common rhetorical trick among people who have weak arguments (and know it) is to throw their pitches at folks who aren’t experts in the field. They pretend like they’re honestly exposing themselves to critical analysis, but it’s a smokescreen. It makes their arguments weaker, not stronger, and if they’re trying to convince you via that method, it’s because something about you gave them the impression you’ve got a lousy batting average.