Sadly, I have seen my fair share of line cutters over the years. I am very understanding of the people that are catching up to their family. It is normally fairly obvious, and the family is not that far ahead.
One father/daughter at Space snuck in to the FP line right in front of us while the CM was doing a kid swap. Back in the paper pass days, I knew they would be stopped at the second checkpoint. When we all got there the father turned to the daughter and said what did you do with the fastpasses, then told the CM she must of dropped them. They were let on.
I told the CM they should have made them go back and find them, because they never had any.
You can choose to get angry (I hate seeing people cheat the system), but I too am on vacation, and want to enjoy myself. However, if Disney continues to allow some of the fragrant line cutting, then everyone is going to feel that they too should cut the line. It is a balancing act, and one I admit, that i am very happy that i do not have to deal with on a daily basis.