This is only my opinion on the subject of playpens which probably goes against how the majority feels, but I am used to being a minority when it comes to many subjects.
I have nothing against those that feel the need to use a playpen. I am the oldest of 10 children. A playpen was the last thing my parents could have afforded. We were often lucky to have milk (real milk) in the family and when we did it was for the baby. I hate powdered milk to this day. lol But I'm getting off subject. "I" often took the place of a playpen as older sister and I didn't mind taking care of my siblings when mom needed to do other things. It was my way of helping out for all my parents did for me. To this day I have a very close relationship with my brothers and sisters that I helped with.
I was born at a time when most parents put a blanket on the floor and a few favorite toys, and placed baby on the blanket. I think this encourages the baby to become more mobile. My daughter has a playpen which she no longer uses because Mckenna is almost 2 years old now and she can easily climb out now. But when she was smaller, her older brother who is two years older than her often climbed in with her. I myself find this possibly more dangerous than having baby on the floor on a blanket. The area in a playpen is more confining than a blanket.
However if the family has pets I can see how having a playpen would be helpful as puppies and dogs want to play with baby. Cats however are different -- the playpen is no help as they can climb into anything.
Too often I see parents use playpens and infant seats as a babysitter, I myself have a big problem with that because babies are very social little beings and they need human contact, hugs and all that goofy baby talk us adults do with them. I have studied on the subject in college and learned that humans can actually die from lack of physical human contact. Also this contact promotes learning in many different skills as baby tries and wants to mimic everything that parents and siblings do.
For myself and regarding my own opinions I find a playpen unnecessary even though I do have two large dogs.
Another thought I had is when my children were at an age that was appropriate for a playpen -- they preferred following me around the house as I did my work. I made up little games to allow them to help. They enjoy helping to put toys back into the toy box, or being given a dust rag of their own to help mom dust the furniture and tables. My two year old granddaughter follows me around all the time and is determined to help me. She even helps me do the laundry. She likes to attempt to fold things up -- sometimes she finds it just as fun to unfold but that is just one of the joys of being a mommy. My granddaughter also always follows me to the washer and dryer when I do laundry. She makes it known that she is very good at placing the wet laundry into the dryer for me -- granted we have to do it one piece at a time this way, but it makes Mckenna happy.
One thing I have learned in my years of having children and grandchildren is that they are not small for very long and the time spent with them as they grow can never be gotten back. I would prefer that my work takes a little longer now and spend that time with the children. In later years (which come more quickly than we realize) we have a lot of time to do things without the help of a curious toddler. I think the social activities are good for the babies too. However I understand that sometimes parents have limitations on their time -- I think it's important for each parent to weigh what is needed for their own families and do what is necessary.
That's my two cents worth for all that it's worth. 
I'm certain many others will disagree with me, but those are my feelings and opinions on the subject.
