I am courteous to those that deserve courtesy. If I see a blinker I will normally let them in. The exception being the person with no one ahead of them, not going the limit and trying to get in the left lane ahead of me to hold me up.
I will NOT allow anyone in who is driving like an a-hole, riding the exit only lane until the last minute and then trying to take me from the shoulder or cutting the line on a busy exit ramp regardless of what they drive.
I have been known to let someone in who is trying to jap me out and as I do, beep the horn and alert the cop who is looking for exactly this type of behavior.
That guy was pissed. I blew him a kiss.