I hate playpens but have found no alternative but to get one. We live in a very small flat where the living space is living room and dining room combined with an open arch way into the kitchen. We have so much stuff packed in that there is no way I can keep LO safe without somewhere enclosed to put her. I hate putting her in there but she doesn't mind (she currently loves pulling herself up on the bars and giggling through them) and I only use it when I need to do something where I can't be right next to her. I found it a lifesaver the other day when I dropped a full glass of Coke onto her jumperoo. It bounced off it, hit a table leg and smashed. Her jumperoo, play mat, changing mat, the wooden floor and the rug were all covered in coke and shards of glass. She wouldn't let me put her down in her cot in the bedroom but she was happy to watch me from her playpen which thankfully avoided the mess. Without it, well we would still have glass and coke everywhere!
To be honest, I don't think it's helpful to pigeon-hole different things into 'natural' or 'unnatural'. I read somewhere that early humans used to swaddle their baby, wrap them up in a kind of baby sling and hang them up from a tree while they went off to hunt etc. I think I'd prefer a playpen to that, but they were as natural as you could get really! When I first realised we needed a playpen I fought against the idea for a long time, but eventually I realised that what is 'natural' or attachment parenting is what feels right to you instinctually and keeps baby happy. They're not for everybody but in our case, somewhere safe to put LO where she can be happy is as natural as it gets really. I still hate the way it looks so I'm currently coming up with ideas to decorate it and make it look more friendly.