Purists say sheepdogs should never play - we disagree!
This is Ezra - one of the toughest sheepdogs we've known. He'll work cattle as well as sheep, but (like his mother, Mel) he's hard to stop. Unlike his litter sister, Carew, Ezra is pushing all the time - often too hard, but if you need some stubborn sheep moved, he's definitely "your man".
Both Ezra and Carew love to play with balls, frisbees or any other toy that's to hand when they're not working, and we encourage them to play, but when we tell them to stop, they stop playing instantly. We also make a point of not mixing work with play. This keeps it clear in the dog's mind when is the right time for each activity.
I think the reason so many sheepdog trainers don't allow their dogs to play is because too much play, can result in the dog preferring to play rather than work. In our experience this will only happen if the dog is allowed to play too often and for too long. As with most things in life, playing in moderation is perfectly OK, especially if the dog knows that when you tell it to stop, you mean it.