It's good to remember that those NPCs are all under DM control, not player, and exhibit the same personality traits that the PC does. Wow, that would be a fun roleplaying challenge for the DM! If your player is argumentative, imagine him trying to give orders to equally argumentative copies.
And if you're good? The roleplaying challenge of trying to convince "yourself" that your belongings and friends aren't his belongings and friends is a wonderful one. Handle it poorly, and your potential ally will be out there feeling abandoned and alone. I wouldn't want that, not if my PC valued his reputation.
It's also worth mentioning that the mind seed ability would crop up relatively rarely. Roughly 1 out of 8 times, with a variable saving throw when it does succeed and no immediate way to tell that your enemy has been affected; I'm guessing that at least a few mind seeded enemies are going to get accidentally cut down in the heat of battle.
Auraseer, I'm willing to chalk it up to campaign differences. My game is high level (15+), I'm fairly sensitive to power imbalances, and I wouldn't have the slightest hesitation about allowing this power or the class. Heck, I can think of so many ways to make the manifester nervous of abusing the power that it doesn't faze me in the least.
Even better, it's a power that creates instant DM plot hooks! There are so many fun and interesting story paths that could come about from this that I'm suddenly wishing someone was playing one in my game. Luckily, though, it's easily house ruled for someone who doesn't feel the same.