I think it's important here to note that the piece of advice in the book was specifically directed at the Dumbo ride. It was not a "here's a strategy to use for ALL rides" thing.
Dumbo -- as it exists today, pre-double-Dumbo-new-Fantasyland -- is a very popular "little kid" ride, and little kids like to do it 2 or 3 times. And the ride is not necessarily conducive to mom, dad and little kid riding together in one row. AND ... the queue is set up in such a way that it's quite simple to have one parent wait while the other hands off the kid. If the child wants to ride "with" both mom and dad (as opposed to "with mom in the same seat and dad sitting behind us"), the best way to accomplish this is to have both parents wait in line at different points in the line. And if parent #2 is 24 people behind parent #1, then the child gets two rides in a row and everyone is happy.
Disney also has no problem with this particular ride strategy because -- as mentioned upthread -- they are not going to load a stranger next to an adult in Dumbo (as the rows are really snug for two adults and they're not going to put someone else's kid in with a total stranger). And the second parent has waited in line just like the first parent did, so no one is "jumping".
On a ride like TSMM, Disney will load "single riders" -- total strangers -- together, so if you are waiting and ride with your child, and then when you end your ride you hand your child to the other spouse who has been waiting, then you WILL be taking a seat away from someone Disney would have put there. So that IS "jumping".
It very much depends on how the rides are loaded, how popular they are, and how easy it is to get from where the ride ends to where the ride loads.