Oh, even by MN AIBU standards this is a ludicrous thread.
I have only one child. He is 11 months. I have a playpen. He spends, on average, an absolute maximum of 15 minutes a day in there,though he's often not in there at all. Sometimes he comes to the door with me, sometimes he comes to the loo with me, sometimes he sits in the highchair while I cook, sometimes he spends a few minutes in a playpen.
I can't believe anyone would consider that immobilising a child in a sling or a bouncy chair is any better than a few minutes with some toys in a playpen. Dress them up as different but they are the essentially the same thing. I have a sling and it still gets used from time to time but it is far more constraining of a mobile child who is practising pulling to stand etc than a playpen. Some of those walker type/bouncy things are actively frowned upon by physios.
You can explore in a playpen just as much as you can in any corner of any room. Ds spends far more time wandering about the place and touching/banging/mouthing/examining random objects than he does with his toys in his playpen. I fail to see how it will have any impact whatsoever on his toileting or language skills that, for a tiny portion of his day, he sits in one place rather than another 
Balance, people, balance. Some posters are clearly letting their imaginations run riot, assuming that toddlers up and down the country are isolated in pens for hours upon hours while their families go about the business of life in different rooms.