I was sad when Yahoo bought delicious 'cause I knew it was over. But Delicious was the first of its kind, and provided really good discovery. I dont understand why pinboard bought it. To preserve the bookmarks?
Probably for the same reason that the Internet Archive decided to thoroughly archive all of GeoCities when that closed. People tend to like preserving what they view as being important history.
Also, not wanting a billion bookmarks to vanish is a good reason. Plus I'm sure he can convert a bunch of former Delicious users over to Pinboard simply by operating the service.
Sadly that ship partly sailed when Yahoo dumped it; a huge proportion of the accounts never got exported including ones who explicitly consented to the transfer.
I don't know anybody who stayed with it after that