[beaned]

Eventually such a practice will bite you in the ass when you form an opinion on something you think doesn't matter, but actually turns out to matter.

[Zetice]

This presumes all of your opinions are wrong, and further presumes you'd hold these beliefs strongly when information changes, which is the very definition of irrational.

So yeah, if you act irrationally, you will make bad decisions. But it's not irrational to believe something loosely based on a very limited set of facts.