It's storytelling. You can't possibly be expected to have genuine passion for the art and science behind every interesting story in the world... Otherwise the only stories that get told will be PhD dissertations.
Stories like the Longitude story can be told outside academic dissertations - by people like Dava Sobel, though, not really by Fred who's just read her book.
It's not a matter of arrogance, pseudo-intellectualism or anything else. It's a matter of wanting to share interesting things one knows.
The phrase "I explained about longitude" paints a very different picture in my head to "I told my friends about this great book about longitude", but maybe that's just me.
If I don't make any more replies, it's because some people who don't like the tone of my original post are taking the opportunity to downvote everything I say.