Her personal interests are important in that they do not align with that of the Zerg or Overmind in general. She is only out for herself, not for the Swarm. Her freedom from the OM influence in BW is a testament to that. The Zerg are little more than slaves to Kerrigan than they were to the OM when originally conceived and that is the point I'm making. I sincerely doubt the OM would want it's children to be playthings to meet the personal whims of a childish individual such as Kerrigan (such as what happened in BW). The OM only allowed her leeway in SC1 because it knew she was on a leash, albeit with a long lead...
Also, I tend to think Kerrigan was given more free control because some individualism was needed as a limiting trait for psionic power to be integrated and work properly in the Swarm. It may explain why normal infestation is not enough to incorporate psionic power because if it was that easy to incorporate, the Swarm would have warriors with psionic powers by now.
I'm not entirely sure it was despairing more than it was relishing it's clash with the Protoss. Whatever the case, the OM does not want to use Kerrigan in the strictest sense - it wants to harness her psionic potential for the Swarm (Kerrigan herself is just a shell really) and it hadn't fuly done that yet. Kerrigan's induction into the Swarm is an ongoing investment to incorporate psionics into the Swarm - at that moment, it had succeeded in bringing psionic power to the Swarm but not to the extent that it could be widespread across the greater Swarm because that would be the determinant in its battle against the Protoss.
The OM did not want to potentially lose something (there were no assurances that, even with Kerrigan, it could win - it just thought psionic power would give an even battlefield and perhaps give it an edge) that could benefit the Swarm in the long run (of which, Kerrigan is the beginning) after searching for so long and almost despairing that it would never find anyway. The OM was trying to cash-in on its apparent luck but not to potentially piss it all away as well - it was "risk averse". How does that not make sense?
Look, this is just the second alternative theory. The Dark Templar on paper seems to fit more neatly. Another thing to consider on this front is that Kerrigan was not really needed on Aiur for the OM to successfully invade Aiur. Maybe the OM changed it's mind on the importance of the "determinant" when it found that only the DTs pose any threat to it, specifically when it mind-melded with Zeratul.






