Totally agree with avoiding pen birds if one has other options. Do it to often, and the dog does learn bad habits, especially that the dog can get right up in a birds face and still get rewarded with a dead bird. Better for the dog to learn he/she put too much pressure on bird and sent them flying with no reward than for a reward of a dead pen bird shot with the dog on point 10' away. Maybe no reward for you - no dopamine because you didnt blast anything - but in the end much better for your dog.
Pen birds and too much pigeon work, etc. and youll end up with a dog that puts too much pressure on a covey and send its flying well before you are in a position to shoot, especially after the first month or so of the season when birds are more wary and better fliers. Then, youll start reigning in your dog, forcing him/her to work closer and closer so that you do have a better chance of getting a shot when the dog gets right up in the birds' faces. And now instead of a big running dog that covers 3x as much ground as you are walking, or more, and pins birds a quarter mile away, holding them for 10 minutes, youve got a dog running under foot, making you walk almost the same amount as the dog. Happens a lot with inexperienced guys that start in NAVHDA - I think the NAVHDA events are set up for and by control freaks and encourage this.
Antecdotally, of course.
And there is me stepping off my soap box.