• Security: contributing code in a wiki and executing it on the server sounds like a very bad idea in terms of security
• Usefulness: why the heck do you need to run arbitrary code on a wiki page?
• Maintainability: There's a reason why code and templates are usually separated, and that is maintenance.

That said, I'm sure it's a very nice proof-of-concept demonstration of what parrot is able to run, or hopefully will be able. And adding wikicreole markup to November is certainly a very good idea.