I suppose the pre-Gen VI Exp. Share would be okay to have in the early game, because it does help with getting Pokemon caught up...
...Post-Gen VI Exp. Share, though, no, will pass. I'm currently skirting really close to the level cap already just before Aya (had to buy Common Candies to get everyone back down to the level 43-44 range, and I'm still worried about over-leveling even then...), and having everyone on my team gain experience would only speed up that process.
Yes, grinding does get rather tedious for me before battling Noel (where I consistently find myself grinding everyone up 4-5 levels, else I risk being severely under-leveled as soon as I set foot on Route 1) but really, that's the only time I can think of, and after that, you're not too far behind. With the level cap, you actually want to avoid leveling up too much, and the Exp. Share would hurt that too much.