If you're an authority in your industry and share new data or strategies as a guest on another industry-related, high-quality, well-trafficked blog, the link in your author byline at the end will always count for a lot. You've earned a vote because you're an authority in your industry. The visits and social signals of people commenting on that post, sharing it etc., will indicate that to Google.
On the other side, if, like Geoff Jackson said, you're throwing together 500 words of rubbish to get posted on a spammy, low-readership blog, just for some exact match anchor text that you've used in links 100 times before, Google will discount that link's value, and might even penalize your site.
I guess my point is - excellent guest blog posting is still very white-hat, and always will be. As industry authority figures share their knowledge around the web on high authority blogs, the links back to their own content are great votes that they've earned.
I interpret what you're saying in your question to mean all guest blogging is bad and Google will eventually throw it all out. However, just as with anchor text, if it's done well then it still counts for something. Guest posting is still a great idea if you're a great guest posting helpful content.