Wouldn't separating the indexable content from the actual DOM representation be a huge opportunity for abuse? It seems like it would make it much easier to game SEO or mislead users for even more malicious reasons.
Search engines have been dealing with black hat SEO for a while now. I'm a bit persuaded that Google knows whats up.
As in, I think it's just one of those inevitable technical challenges of castle defence.
Build better walls, they build better trebuchets, ad infinitum.