It can be very harmful to act on an uninformed opinion. There's also simply too much out there to be informed on everything, so logically there should be lots of things you aren't able to have a valid opinion on.
People frequently get around this by finding people they think they can trust, and borrowing their "informed opinion" on things. But this can also be risky, and easily leads to groups with highly polarized opinions (political parties, etc). Even borrowing scientific/expert "opinions" on things can lead you astray, as we've seen with many of the funded studies on food health. Two experts can easily have conflicting opinions on something, with strong arguments/evidence to back up their stance.
So basically having completely uninformed opinions is dangerous, it's not possible to learn everything well enough to have a good opinion on it, and borrowing others opinions on things only works some of the time. So it's probably best to accept that you shouldn't have a strong opinion on most things, and to be always willing to re-evaluate your opinions if you run into evidence that refutes your current opinions.