We always use a pen if we are staying more than a day in a Cg and haven't had an issue. We have two small dogs and they know that the pen is the spot to "go" which makes it easy for us to clean up. It keeps our dogs safe from other dogs that snap a leash and attack them. It allows them the freedom to wander within the pen while we sit in our chairs and relax. We also enclose our steps so our two cats can come in and out of the RV. The cats are leash trained but neither is fond of strangers so I prefer to have the leash on them and the door open so they can scoot inside when they need to. The pen stops them from going underneath the rv and slipping away after a squirrel.
Personally I feel that someone who goes to the trouble of penning their dogs are very responsible and they are ensureing that their dogs leashes don't scrape up the ground or local flora and fauna. Much easier to ensure all is scooped because its in one spot and it allows the zoo to stay with the family.
Any CG that says no to a pen we do follow their rules but we try to avoid those cgs.