Dark Eldar and Eldar

I was wondering how useful using Eldar allies are to battles and what are the best units for it.

Mushkilla will probably tell you that Eldar are not required to have a valid tournament army and he will have some valid points in that.

However, if you want to be top-tier, Eldar will make it easier and will let you largely improve your chances of winning. Farseers are a great addition. So are wave serpents and windriders. Warp spiders are amazing, too. All in all, depending on what you want to achieve with your list, our craftworld cousins can be a huge help.

That said, you chose a wrong section of this forum for your question and fully expect this topic to be moved soon.

Moving topic to the tactics section.

A similar discussion was had in the topic linked below (a few weeks ago).

Are Allies Really Necessary for Dark Eldar to be truly competitive in today's environment]

Hope that helps.