I like being able to pre-board. When we had just one child, I would leave the baby with DH while I pre-boarded to install the car seat. Now with 2 kids (1 yo and nearly 5 yo), that is a bit tricker and I think its just easier for all 4 of us to board at the same time.
My biggest issue with not being able to pre-board (or boarding after elites, 1st class/ect...) is that is so hard to walk down the airplane isle carrying a car seat and a diaper bag without bumping into the people sitting on the isle. I try to carry the car seat above the level of the seats and my diaper bag is usually a backpack, but still its tricky. And, if I leave DH to board later with the kids, I'm going to need to carry some of his stuff too.
Boarding the plane is one of the most stressful parts of flying for me. I think I worry too much about getting in other people's way. If I'm not with DH (who is super impatient), I'll won't even attemp to de-board until nearly everyone else is off the plane.
I've had pretty good luck getting allowed to pre-board - when I ask.