I think we need to realize there's a distinction between line cutting and stepping out of line for whatever reason.
If a family of 4 get into a line and then realize 10 minutes (or 30 minutes) later that someone needs to go to the bathroom or grab a drink, then that doesn't really bother me. More than likely, the people behind them will see them leave and then get back in line. Someone doing that is not causing a problem. They were there from the beginning, stepped out, and then came back. In other words, they didn't affect the wait of the people behind them.
However, if someone gets in line and then other people join them later for WHATEVER reason, they are affecting the wait time of the people behind them. Sure, one or two people are not going to make a difference, but if it was a standard practice for a lot of people, it could increase how long people wait. It's also not fair for the people that waited patiently in the line for a group to catch up others in front of them after sitting in the shade or riding another ride.
This really comes down to common sense and consideration of others. Just think about how you would feel if the same thing was done by someone else. (Sure, some of the posters in this thread say they wouldn't
Extremely well said!!
To tack onto this... I think it's important to note the difference between a family that plans to allow the cutting and the ones that it just happens to. If you and your family get in line and 30 minutes later your child has to go to the bathroom, I see no problem with hopping out and back in line.
However, I do have a big problem with the families that know one or more of their group have to use the bathroom and the other members go ahead and get in line.
If you know part of your group has to use the bathroom, I think it's just common politeness to have the entire party wait until they can join the line at the same time.