I know not all religious people are stupid. A brother of mine is actually a very intelligent person, well above average, and a "died in the wool" believer.
And for the most part, in a perfect world, you are correct on the second bit as well. Normally there isn't anything wrong with believing in whatever makes you happy, as long as it does. True enough.
But, the problem comes in when your exposed to other people. Belief without proof by its very nature sets you up to be misled by others. If you can't hold those in charge accountable for their actions and decrees, why would you go along with it?