What are the advantages of encouraging a creature to be "playful"?

[InspectorMonocle]

So, apart from making it easier to teach your creature to catch things and throw it back (which is really cool when mastered) what are the benefits of having a playful creature? Besides making him physically more fit and keeping him busy for a bit I don't see a good reason to encourage him to be playful? Also what are the disadvantages of a creature being playful? And is it recommended for a evil/good creature to be playful?

 

[Gremxula]

Playful creatures tend to pay more attention and so are  easier to train.