I agree. A worldwide leaderboard becomes most important to only a small % of the top scorers. A friend leaderboard becomes most important to only people who have any amount of NNID friends and followers. You leave a majority of players out. By bringing in Miiverse posts of people who have scored similarly or close to where you are can make a player of any skill feel like they have a shot at competing even if they really don't against the world at large.
Keep in mind we're talking about a game where some of the challenges are seconds-long. A leaderboard for each would cease to be meaningful there, really.
A better approach might be something like Pikmin 3's leaderboard where they show you a graph and where you land on that graph but you're not scrolling through thousands of pages of people's names and scores and can get a decent idea of how you compare versus the world. Again in the challenges where it's mostly fractions of a second this might not be interesting or useful data.
Look at it this way. Competing for the high score on an arcade machine at a local pizza joint is interesting because almost any player who tries the game has a shot for a single-digit spot. It's a small pool. Putting your initial in feels like a real win. With internet leaderboards, 95% of players won't have a chance in hell and may get discouraged because of it.
I feel like this is in line with their bias towards couch competition versus online play too. Backwards as it may be.