Like pretty much everyone else here, I've encountered this regularly. For my own part, I believe that you should take the right of way according to the rules 99% of the time. But for your consideration, I'll offer this key exception where I think it is good to cede the right of way: in situations where traffic is busy enough that the only way the other car can possibly get out is if someone lets them out. Obviously safety needs to take first priority so this shouldn't be done in all cases; but if its safe to do so, and you can tell that the car waiting to turn or merge is likely going to be stuck for a while if no one makes a conscious choice to let them out, I think it's worthwhile.