Hello! Great question; I've worked with Magento quite a bit and layered navigation can definitely be a bit tricky from an SEO perspective.
The most important thing to opening up some or all of these pages to Google for consideration, is having unique content on the page. This is not something Magento themes typically include by default but will be 100% necessary to get most if not all of these pages to rank well.
Most typically we add a static block to the top or bottom of the page to add uniqueness. Without it, all of the content on these types of pages are almost always 100% non-unique and from my experience Google ignores.
If your layered nav pages also use pagination, you'll need to make sure you follow best practices for using canonical and/or rel prev/next/view all to make sure Google isn't looking at your paginated pages as extra duplicated content.
And as a final reply, I think it can be a great idea with limited consequences as long as you follow the above advice regarding unique content and pagination. Additionally there's no worry regarding Pagerank or destroying the world. 