Hemingway once said that every new language is like a new world opening for a person. I truly believe he was right. In that respect every language that you study expands your vision and in fact increases the amount of information that you have , from her it is just a short step towards being smarter. Of course the question is how one determines who is smart and who is not - measuring intelligence is still a problem. But from my personal experience bilingual people are more adaptive, more knowledgeable , they easier resolve problems ....
Which could be explained by you meeting bilinguals in environment which suits this situation, while you presume that thick cashier down at Alepa is monolingual without actual evidence of it.
Human gut feel generally is not very good, we tend to notice exceptions, specially when they reinforce our expectations. We often "realize" that we are "always" in the slowest queue in the store. In reality, when observed objectively, this is not true but our own perception cheats us. For example because we have more time to observe our surroundings when in queue which is going forward on snail pace.
I also recall one study which pointed out that bilinguals have higher chance for multiple personality disorders.