[Ithekro]

Spoiler for Nagato:

I see. Though I would point out that being able to use Haruhi's powers is not enough for the Data Entity...They want and need to understand her power so they can use it for their own purpose. They don't understand why Nagato did what she did...it seems beyond their reasoning (since it was classed as an error). They might not even know how she did it because of that error.

It is not enough to just have a power. If their goal is to "evolve" with it, like Nagato states back in the first season, they need to understand how it works, and why it works, in order to actually evolve.