I accessed the Dream World a few times, but not frequently enough to get all of the DW Pokémon or get the medals in BW2 for using it enough times. Frankly, I thought the Dream World was kind of a hassle, and the games and activities were repetitive and frankly not enjoyable. Add that to the frustration of playing them on a less-than-perfectly-responsive laptop touchpad (rather than with a mouse), and they were mainly just a chore. Overall, it seemed like the generational gimmick and an excuse to use the games' internet capabilities. I'm particularly annoyed that the Dream World was up for so short a time after the BW2 release--I would have had to be very on top of things to get all of the Dream World medals. Moreover, it's annoying that the game sells the whole Dream World thing during gameplay as much as it does, especially now that it is useless (and wifi is useless too, but that's another complaint). I was recently playing a Black file for a challenge, and the Dream World thing comes up a lot at the beginning of the game, and wastes a lot of time to activate something that's now essentially useless.
As for unreleased hidden abilities, I think Game Freak just likes to leave options for more events, special releases, etc. They tend to avoid giving us everything at once, and that is an extension of this. Plus (though this may be an unintentional consequence), it keeps us guessing and discussing the games (as we are here), building hype for newer releases in the process.
A random comment: I feel like Game Freak has been looking back at random lines of dialogue from NPCs and basing new features off of them. Munna was a pink Pokémon with a floral print, as mentioned as one NPC's desire in an earlier game, and when I was playing through an older game the other day (I think Ruby?) a researcher made a comment about trying to understand Pokémon's dreams (and that it wasn't going very well at that point). So I think some of the gimmicky features may be a combination of what works well with the newest technology (e.g., internet connectivity for Pokémon) and what subtly references comments made in previous games.