The Tyranids would like to know your location. And since our infested can control the living, the dead, and with just a little tweeking, technological units; I'd say they would be right at home in 40k. I do agree the tenno would be doomed though. Even if it was death through a thousand cuts.
As some others have said it's probably a courtesy thing. Would you want to talk to some guy hiding in a tank when trying to hire him for a job? Or would you rather see his face when talking to him?