The Hobbit, as a trilogy, falls into the second category. The original novel, as prequel to Lord of The Rings, contains only fleeting connections. There is the scene with Gollum and Bilbo's discovery of the ring, and the presence of Gandalf. From this Tolkien was able to create a rich tapestry fantasy world. But The Hobbit, as a novel works on its own. The story is succinct but still has a number of subplots, most of which are resolved adequately without the need for appendices. For those who came to Middle Earth via the LOTR trilogy, reading The Hobbit as a prequel is stil a rewarding experience.
The films weren't comfortable with mere fleeting connections. Without being cynical as to why it took nearly nine hours to tell a story of less than 300 pages, it is worth considering how effective the films added material to build a coherent six part story, from Unexpected Journey to Return of the King.
The primary story of The Hobbit follows Bilbo Baggins as he aids Theron's company of Dwalves in reclaiming the Lonley Mountain from the dragon, Smaug. There are three additional subplots added to this: the white council's investigation of the necromancer, the elf-dwarf sexual tension, and the orcs' pursuit of Theron. The white council stuff sees Elderon, Sauraman and Gandriel return to the series to create a link to the LOTR trilogy. The elf-dwarf romance adds a romantic subplot but little to Bilbo's story (hell, it's not even clear if either Bilbo, Theron or Gandalf even know about it!) and allowed Peter Jackson to include Orlando Bloom again. The Orc subplot does enrich the battle scenes and provides the orcs chasing the company with some purpose. This subplot also has nothing to do with LOTR.
The white council parts are essentially pointless and distract from the main story. For viewers it is nice to see familiar faces and, for those who pay attention, it is nice to see that they knew of Saurein'a return 60 years before Gandalf visited Frodo to get the One Ring (now there's a plot hole for you). But it is The Hobbit's version of including C3PO and R2D2 in the Star Wars prequels. Familiar faces are fine but they create plot holes and actually take the audience out of the story. The distraction from the main story isn't a sign of good story telling - it's a sign of trying to serve too many masters. Even though The Battle of Five Armies is the shortest of the Middle Earth movies, it still spends time dealing with subplots that set up a separate trilogy - one that runs between nine and eleven hours depending on the versions you watch. Due to the excellent prologue in Fellowship of the Ring, there is little that needed to be set up anyway. The Easter egg in this case has developed into a significant number of extra scenes, none of which serve the story, and only serve to prolong the length of the film. (Someone there's even more for the extended additions!) Still, this is still better than Star Wars: Episode I - The Phantom Menace, which pacts so many Easter eggs and video game scenes into it that it has little, if any, connection to even Episode II, let alone the original trilogy it is a prequel to.
Sequels have been pushed into this problem too. The need to include a larger story sees stories that full of characters with little time for the audience to connect to them. Iron Man 2 sunk from the need to include scenes fleshing out SHIELD and the Avengers, at the expense of their antagonists and sidekick dynamic. XMen: The Last Stand and Origins: Wolverine both loaded themselves with characters and tried to fit into a larger story, creating plot holes and leaving themselves with little room to even tell their primary story well. It's worth noting that the First Class X-Men films, The Wolverine and Iron Man 3 cut down on the number of characters and subplots and told much tighter stories that ignored the overall continuity with a preference to telling their primary story as well as they can.
A rare exception to all of this is the excellent Captain America: Winter Solider, mainly because its primary story is one that pushes the larger universe forward. For prequels, this just isn't possible. The audience already knows how it is going to end. Better off exploring a different part of the world, as X Men First Class and the Hobbit novel does, than telling a direct story that we know the resolution too (the Star Wars prequels). It will be interesting to see how the forthcoming Pan (a darker, grittier prequel to Peter Pan) handles these different demands.
In the end, whilst there are some cool moments for fans of the original triligies, prequels would be best if they kept the references to future stories to a minimum. Easter eggs are fine, but not when they don't serve their true master - the primary story they're trying to tell. I don't think the audience is even crying out for these parts of the story to be told - most likely they just want the story to be told well so they can lose themselves in another world for two (or three) hours.