Just opening your cellphone with its hideously bright screen will make you an "annoying moviegoer" to many people around you.
Look at it this way: If I pull out my phone to use Tattlr to report a person chatting it up on his phone behind me during a movie, I'm simply piggybacking on the existing disturbance as opposed to creating my own disturbance. Who's going to notice me using Tattlr when the guy behind me is being a far greater nuisance? Everyone's attention will be on the jackass talking on the phone, not the jackass tapping the screen on his smartphone.
OR
If a shouting match breaks out in the back of the auditorium. Will people be more worried about getting nachos and soda thrown on them or the guy 2 rows up illuminating the place with his smartphone?
I, like you, can maybe think of one other bad experience I've had at the movies before this weekend. Tattlr certainly wouldn't be an everyday kind of app. Heck, one may only use it once or twice a year. However, I think there's value in the peace of mind of being in a large dark room knowing a theater attendant is only a couple screen taps away.