But how do you know whether the text is important without pulling out your phone to check it?
Movie etiquette is to not use your cell phone in the theater. Whether someone's said something or glared at you, I can assure you it's been a distraction for people sitting behind you, because the sudden light, no matter how dim, is going to draw an eye. Like Chris said, if it's so important to check your phone during a movie, you probably shouldn't be in a theater. People are responding in a hostile manner because you're admitting to rude and selfish actions in a public space and then saying that, because you do it discreetly it's not a problem. I guarantee you that's not the case and that there's very rare moments where one should have a phone out in a theater (those moments mostly have to do with calling the police for some sort of activity in the theater.)