I disagree. Pinterest is different from other social networks in that it is about collecting content. With content being your secret sauce, you don't want to give it away for free. Anyone can bookmark an image, not anyone can find the perfect image. Pinterest gives you that super power.
It's common, but I still don't like it. I deleted my Facebook last year and it was shocking to me how much "public" content Facebook hides behind a login requirement. The same with Quora, and apparently Pinterest. They make it look like a public page but after you scroll they're like "uh-uh. We need your personal data if you want to see the rest."
It really soured me against these sites to see the way they take other peoples' content and lock it behind a wall.
I just walk away when I see that stuff now. I miss the old internet that felt more like a public park than an amusement park.
Casual visitors not interested in actually joining a website (at that moment) can be of value if you don't actively antagonise them. Saying that you need an account to view the content when someone follows a hyperlink posted somewhere to a Pinterest board means that you will turn away a decent percentage of those visitors with a sour taste about your service and brand. Sure, there is the old adage that there is no such thing as bad attention (any attention, good or bad, builds your 'brand'), but I'm not convinced this works in this case.
To illustrate; literally the first thing that popped in my mind when seeing this topic in the list was “Ah, that silly imageboard you can't view without signing up.”