I completely agree that I do not like the practice of line cutting, but what the UG outlines is not exactly how it is being interpreted here.
It's mainly for rides like Dumbo, Aladdin's carpets, etc, where the queue is exposed and tight. These are start-and-stop cycle rides.
How the "hail mary" thing actually works is like this :
1) One parent gets in line with small child.
2) Parent two waits a minute and then gets in line themselves (with at least one "cycle" of people in between parent one and parent two)
3) As parent one gets off the ride with child they pass the small child to the parent already waiting in line (easy to do on something like Dumbo, this isn't like jumping the queue at Pirates or Splash)
4) Child gets to ride again with Parent Two, who was already in line, while Parent One continues out and waits at the exit.
In essence, the child gets two rides. However, on those cycle rides the parent number two (who also waited in line) would just get their own elephant/carpet/whatever anyway, so no one in line "loses" their place in line by letting the small child ride again in a car that would have been occupied by just their parent anyway, instead of the parent and the small child. Just as many people are going to get on that cycle as would if the small child had not gone again.
Line cutting is just a bad thing, and this does seem like "training" kids to cheat the system, I agree. However, the ACTUAL hail-mary pass as described in the UG (and I've outlined above) is pretty harmless when it's a kid 3 or under, and both parents have waited in line. I don't see (again, on these cycle rides) any problem with it if the parents are smart enough to do it properly.
Those people who need to find their families in line and want to cut the entire "Splash Mountain" queue? Sorry kids, meet them at the exit...half the time they are lying anyway.
NED