I know the majority of people don't let google crawl their WordPress attachment pages.
In my opinion the attachment pages aren't duplicate content because they don't show the whole content but just a unique image or PDF link or other file. I don't understand why people hesitate to use them in a natural way.
Although it will cause Google to index a lot of extra pages in your site, doesn't this increase the visibility in search results? What are the drawbacks of letting Google see these pages?