Just because the kid isn't penned up doesn't mean they need undivided attention. Let the kid explore and hurt herself, it's all part of learning boundaries. I have gates to keep her away from stuff she can really hurt herself on.
Plus, the climbing. If the toddler is docile enough to play contently in a playpen, you don't need a playpen. If the toddler isn't - your afternoon is probably going to consist of a spirited round of "inventive ways to keep my kid in the playpen." Complete with lots of screaming if you manage to win.
