Yeah, I don't really understand the "answering the door" justification for playpens. I just carry my DD with me when the door rings.
I think one of the objections to playpens is that children learn so much from imitation and following you around the house. So when I am doing the laundry my DD (16 months) helps me sort and put things in the drier. She also "helps" hang it out. Cooking can be difficult but I put her on one side of the kitchen with some toys and she watches me from there. She has learnt not to come close to the oven (and the phrase "Hot! Hot!"). I think it is very important for children to see how food gets prepared rather than it just appearing on their plates.
As, the same goes for going to the bathroom. Children learn about going to the toilet through, I am a bit embarrassed to admit, by watching their parents.
If a child is in a playpen while you do chores etc then you also diminish your opportunity to talk to them and for them to develop their language skills.
So I think the further objection is the same as is given to reigns - it deprives children of the ability to learn to self-regulate because external barriers are put to them having to do so. We have never needed a playpen because my daughter has learnt that she is not to touch certain things by me teaching her that rather than simply putting them out of reach.
Of course this has its limits - I wouldn't leave things extremely dangerous (such as boiling water) in her way and simply tell her no. But then I don't think the majority of living rooms have such extremely dangerous things that a playpen, rather than other forms of discipline or else taking them with you when you leave the room can prevent.