How is this different than Adobe Reader, where the ability to execute code within a document reading application has resulted in world wide exploits of operating systems?

If my document reader can execute any code in any language, then any document that I read has the potential to execute malicious code on my computer, and I now have an exploit vector that I need to consider when downloading documents & opening e-mail attachments.

I understand that the code can be sandboxed, but before I implicitly trust the sandboxing technology, I'd have to see an example of an unexploitable sandbox. I don't know of any - but that doesn't mean they don't exist.