I find the fact you mention Wikipedia and Pinterest in the same breath surprising. As someone who doesn't really use the latter, I always knew it as the site to exclude from search results. Whatever the query, a result in Pinterest will look like utter spam that only used my query to get into SERP and then try to make me go to something barely related. On the other hand, Wikipedia seems to be always on topic and usually high-quality content. I understand that one medium is more vulnerable than the other. But still, with somewhat similar content creation structure, I'd expect more similar outcomes. So, what is the main reason for the perceived quality difference?
They have different goals, Pinterest's goal is to let user create collections of things, Wikipedia's goal is to be right.
Pinterest is useful if you are looking for ideas of things that look similar. If you want to choose a pillow, just type pillow and you will find a list of collections of pillows. It doesn't make sense to show Pinterest results in Google because that's not what Google users are searching. The fact that you want to block Pinterest is just because Pinterest is sucessful and has overflowing google images has a SEO strategy.
Ideas as in "search some term, get a not-even-vaguely-related picture collection you cannot see unless you log in"? Or the "ideas" that also happen to hide where that very idea came from?
Pinterest is scourge for a text search engine. It's also scourge for an IMAGE search engine, because it's just hiding the real source.
Pinterest is in the top 10 websites I would blacklist from search results.
