One could make an argument that, in the world of Warhammer 40K, keeping your mind barred and guarded is actually the most rational thing to do. Because if you do not, then instead of saying things like "only in death does duty end", you'll find yourself saying things like, "maim kill burn MAIM KILL BURN" and "Arrghbllgghhayargh NURGLE". Only it wouldn't be you saying those things, precisely, but a daemon that slipped into your unguarded mind and took up residence in your body.
It may be that xenophobia is a local optimum for humanity in 40K. But technology is explicitly mystical in that universe. Imagine how many fewer problems they would have with their enemies if their stuff all worked, and they had more of it.
It's like bringing a 1000 pt army to a 500 pt skirmish. Every time.
Here's the new thread for posting quotes, with the usual rules: