personally, i used them with both my kids. they say not to use them, but they say also not to use blankets and stuff, too. i think it's b/c if you don't swaddle the baby they could get tangled in the blanket and possibly suffocate. i think it's the same reason for the bumpers, so they don't wiggle up to it and get their head stuck with their face into the bumper when they can't even pick up their heads yet. but, when my kids were newborns and swaddled, they didn't really move around in bed so i didn't see the problem, and by the time they could move around enough to get close to the bumper, they could move their heads around well on their own so i didn't worry about it. besides, my kids got up a lot during the night so they didn't really have time to move around that much!
a couple of times when i was washing the bumper, my son got his ankle stuck in between the slats of his crib (he had extremely chubby legs) so i used the bumper to keep their legs or arms from getting stuck, also.