I don't see the point of the friend safari. I mean it is a neat feature but it comes in the post game and frankly between the sheer amount of Pokemon in the game and the frankly tiny amount of stuff to do post game don't see it being very useful.
The point of the Friend Safari (which is basically the game's Safari Zone) is, as always, to get Pokemon not normally available in the wild. It's also the only way to catch the other Kalos/Kanto starters you might be missing (all their second stages have a chance of appearing) without trading.
And, yes, it's post-game only. But hey, one of the goals of the games is to collect all the Pokemon (available), so there's that too.
Also, from what I've read, there's data for three more legendary Pokemon in the game, so there will probably be some downloadable event for each of them.
There's also the Battle Maison, the Battle Chateau, which includes rematches with the Gym Leaders, the extra legendaries like Mewtwo, Z-Guy (can't remember name) and the Kanto birds, hunt for Mega Stones, getting the customization items...