I'm just gonna put in my two cents here.
Up until my last visit to DL I was fine with maybe just one or two people catching up with their party. However when we were in DL in May it seemed to be a huuuuuuge problem. Generally though, it was never a sensible adult, it was almost always a teenager/child. The first time I saw it was when a group of over 12 teenagers walked through the empty section of the Space Mountain line to join two people already in line, and no one did anything. We were to far back to say anything to them, but that really irked me that they got away with it. The cutting that really threw me over the edge was when we were in line for HM (that same night) and these two kids come running through the line. I didn't see them at first, but then they started jumping over the bars to get through people and to see how far up they could get. One of the kids who was jumping over the bars kicked my elbow pretty hard, fell to the ground then kept going to the front of the line! I was shocked! He didn't even apologize until my boyfriend grabbed him and dragged him over to make him apologize. But yeah, the whole time we were there a bunch of people would push and shove their way through, without even so much as an "excuse me." I'd be fine if someone was polite about it, but ugh. It seems like people just throw their manners out the window when they go to DL.