It's not exactly abnormal for Game developers to be fans of other game companies, and play those games regularly. In fact, it's one of the best things a developer can do. Exposing yourself to how different games approach the same concepts/mechanics is invaluable. It's similar to how reading tons of books can make you a better writer. Sometimes, you might find ideas to apply to your own game, other times you can learn to avoid mistakes other developers have made. I read an article about the creation of ARR, and one of the first things Yoshi did was have the design team play a variety of MMOs to understand the market better.
Yoshi P also probably...just enjoys playing Blizzard games. Going to Blizz Con as a blizzard fan is a no-brainer. Note, that he didn't have a guest badge, he had a fan-pass instead. Hopefully he had a great time.
