One problem I've noticed is that those merging are often looking straight ahead when for those in between situations, they absolutely better be stepping on it or yielding while looking (or at least aware of what is happening) over their shoulder.
This past weekend I was driving home eastbound on I-70 in Central Missouri on a four lane stretch (should really be 6 lanes from KC to Columbus) and several cars were passing a couple semis on the left while driving slightly uphill and the front semi had nowhere to go as a car was putzing along while merging. I was maybe a quarter mile behind the semi and could see it unfolding and thought "Hmm, that car merging better get on it." They didn't and jerked the car at the last second to the right and slipped off the shoulder. Luckily the angle of the ditch wasn't too bad, there wasn't a guardrail, and they recovered enough to be on the frontage road (that had no one on it). Could've been an easy death or two if they hadn't realized at the last second they couldn't squeeze in.
Sometimes I wonder if people simply can't crane their neck or don't use mirrors properly. It's much easier to drive when it's all in front of you but checking blind spots and merging is trickier.
Was the front semi driver an asshole? No, had he or she slammed on his breaks or veered over, it would've been a sure multi-vehicle collision as the driver likely couldn't accelerate too much with a low grade hill.
If you can get over while someone is merging, better to be overly courteous than engage with potentially clueless drivers merging. Or in the least, use the horn. Americans view it as the essentially the middle finger, but it still works to send a message of warning.