It makes a difference if everyone wasn't in line in the first place, because it means people who are debating about getting in line are getting false information.
For instance, imagine one person is saving a place for three other people. Now imagine that one person multiplied by ten in a line of twenty. Anyone debating, "Do I want to get in that line?" thinks they're only twenty people back. But then all the groups of three start streaming in -- and suddenly they're fifty people back. That means they've going to have to wait
over twice as long as expected.

That is an extreme example, of course, but in a longer line, it doesn't take nearly that high a percentage of people holding a place for someone else to give the people debating whether to stand in line or move on the wrong information.
People who are going to ride the ride however long the wait is may not care; people who hate lines or hate crowds or who for whatever reason do not get in lines where they're going to have to wait that long are going to be really angry, and justifiably so -- they've essentially been lied to by people who were off doing the stuff the line waiters would rather have been doing, and
would have been doing, if they had any idea the line was that long!
If only one family in a long line has the attitude, "who does it hurt?", then maybe it's not a big deal, But if you're the third or fourth "who does it hurt?" person pushing through a line (or the third or fourth that day), someone may blow their stack. Hopefully they will just glare or tell the line cutter they're being a jerk, but if they do, line cutters are kidding themselves when they think the other person's over reacting. You only get so much time in your day, and if someone does something to take time away from you, that time is gone forever. Wasting someone else's time is rude because you're taking something from them you can't give back.
Not everyone has the same time awareness, but a lot of people who don't usually have the "time is money" attitude in their everyday lives,
totally feel that way while on a Disney vacation! You only need to see one, "Quit crying! I paid a lot of money for this, and you're going to have fun!" adult meltdown to know that.
But remember that it may have stolen time from other people. I always tell my kids that if you're doing something that would cause problems if everyone did it, then they're out of line. In this case, out of line in a literal sense.
Litter builds up one piece at a time; Line Rage builds up one Line Cutter at a time.