Recently one of my characters was involved in a really difficult grapple, and the creature I was grappling was acting after me, then the rest of my party after it. I had it pinned with some good rolls, and was in a dangerous situation mid fight. It behooved me to have my party kill this thing and I did not want to chance this things escape in close quarters. So I delayed. Now, it occurred to me later that it would be rather mean to attain a pin on someone, then just delay for the rest of combat, keeping them pinned until they could free themselves (my character has really really high CMD, making this very unfriendly). This just doesn't seem very nice, but I cannot locate any sort of thing that would prevent this.
You have to make a roll to maintain a grapple every round on your initiative. If you do something other than maintaining the grapple (like delaying), you cannot maintain the hold.