As I see it, this continues the metaphor of the Monestary. Didn't you notice? Each of those animals are the cover art on O'Reilly books commonly discussed here.

I think it also neatly sidesteps the issue of Christian overtones to the existing metaphor.

Sure, experience points are a wash. But, if people enjoy them, why not?

Also, people respond to goals. I'll bet even merlyn does better when he has a deadline or a target to hit.

Besides, I think it's funny that most of those animals grace the covers of books he's had a hand in. It's a reference back to the the language, the community, and the people who've helped it succeed.