I have 2 children--one who will be two the next time we fly and Kate who is 6 months. I am willing to try the new policy once, but I really doubt it will survive the holidays. And I think my situation will help explain why. At 2, DS needs his own seat, which means he needs his carseat. His carseat is a Britax Marathon--its an FAA approved seat which means SWA has to permit me to use it as long as it fits in the seat which it does (barely but it does). However, that thing is huge, and unmovable once the flight takes off. I babywear my daughter and she's safe in a sling so we board her as a lap child--that means we won't be able to board with Group A. By the time we board, the window seats will be taken up. If someone isn't nice and no one volunteers to move when we board then DS car seat will be in the middle seat blocking the hapless person sitting in the window seat. That person is stuck for the entirety of the flight. (I think on seeing the carseat hopefully even a curmudgeon would move but I can't be sure). It takes a while to strap that darn thing in--I'm presuming that B wouldn't start boarding til families were done so that isn't really that beig a deal.
Also consider, baggage. Parents need to pack a lot of stuff to keep their toddlers happy--toys, food, formula, diapers, wipes. Parents who can't preboard will be holding up the line while they stow their stuff, and the other passengers better hope that there's actually space near us for us to stow our stuff. Because we pack 2 bottles in the diaper bag and the formula can in the carryon in case of emergency (it amazes me how often those emergencies have arisen). If we're landing, and she's chowed through her bottles but there's another 10 minutes before we're down, Kate will scream the entire time. Same if DS gets tired and wants Goodnight Gorilla that's several rows away while the beverage cart is rolling. Add several families like us on a completely booked holiday flight and everybody is going to be pretty miserable.
On airlines with assigned seating, preboard isn't as essential b/c we can select our seats ahead of time to ensure that DS gets a window seat and that we're in the first boarding group (most airlines load the back first even though its the least desirable) and able to stow our stuff directly overhead..
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