I disagree with your perspective on front packs; the Baby Bjorn was the easiest, most comfortable way to carry my son from birth until he was walking, which was 9 months. After that, stuffing him into any kind of pack was just not happening. It was stroller or him walking from that stage. I did try a sling once, for about 20 minutes, and that thing hurt like blazes, and yes, the store clerk did say I had it on correctly. It just compressed the nerves on one side of my neck.

We borrowed a Kelty child pack for one trip, and it was strictly for the trail. I can't imagine wearing something that bulky in a crowded situation. That's the sort of thing that can get parents labeled "selfish" "rude" or "inconsiderate" because you just can't judge how far that thing extends beyond your body and you tend to bang into other people. At the farmer's market there always seems to be someone clearing a swath through the crowd with one of those, banging into people and knocking things off the tables.