Bring back wooden playpens!!! Call me old-fashioned, but wooden playpens are great for sitting and crawling babies. My mother, aunts, and grandmas used them. We learned to pull up, stand, and walk sooner. We were safe while Mom cleaned, ironed, or sewed. My sisters and I utilized them for our crawling children, but we seem to have been the last generation to have done so. Playpens are not prisons!
Soft-sided, mesh portable crib/playpen/play yards do not promote standing, balancing, or walking. Since they began being used, children have been walking later, many not until they have passed their first birthday.
The folding kind with a padded bottom is the best. It allows a comfortable place for a crawling baby to sleep and play, so cleaning or cooking can be accomplished. It can also be used outside in the shade while one gardens or reads a book/magazine. Or, of course, while using the computer.
Acting more like a fence, I would think that a bottomless playpen would be easily tippable or movable, which would not be a good thing.