I hate drivers who do this, I don't get why you'd be a dick for the sake of saving yourself 1 second.
Sat in stationary or slow moving traffic, if there is someone waiting on a side road to join I would always let them in, I can't go anywhere and I need to leave a gap for people in the opposite direction wanting to go up that road anyway so why not? After they'd gone, id move up and I imagine the person behind me would then let the next person in if anymore were waiting and so on...
Yet I see so many drivers speeding up on purpose if they see someone trying to join or not letting cars in when they can't go anywhere anyway. What's the point of being a cunt? It saves about 1 second of your journey!