I am frustrated by pinterest because I often use google image search to not only find an image, but to find information behind it. For example I was looking for images of camper van builds - if I saw a particular photo of a camper van interior, I want to know more information about something I see in it or possibly more photos from the same build. The photo on pinterest clearly did not originate on pinterest, it's taken from elsewhere on the internet (probably violating the copyright holder's rights in the process) and doesn't link back to the source in any way. So I'm left with pages full of images that don't help me in any way because they're scraped from other sites and pushed potentially relevant results to the background of search results. The whole process is just frustrating, and to me that's why pinterest has made the internet worse.
Also dismissing the nag screen as "not a problem for most people" completely omits the fact that it's still horribly annoying and continues to show constantly even when i've dismissed it a thousand times in my life. i'm not going to reward crappy behavior and clogging search results by registering for your site, and even if i did, i don't want to have to log in on a million devices and any time i use someone else's computer to avoid seeing it again