Actually, even when you don't have a reactive dog. I can't stress how important it was for me to learn about George's body language and vocalizations during play, whether it's with me or with his dog buddies.
It is especially helpful for me to know what "interest" or an "I'll pass" looks and sounds like when another dog approaches.
Play has so much value in "training', building relationships, learning bite inhibition, impulse control...the list is endless.
Not to mention the fact that it feels great to laugh and find playful ways to communicate and have fun! You get to see a dog's sense of humour come out as well.
The flipside is you learn how quick it can turn into over arousal, trouble right around the corner.
I made this video for my puppy socialization class but wanted to post it here as well. I love Marc Bekoff's article on play from Scientific America. The-ethical-dog