As to why you would do it, it seems that an Asura Cryin’ has increased power. [SPOILER] Not only do you have the power of both the Asura Machina and a demon, but it seems that having the two of them increases the power of your Asura Machina even further as the demon adds power to it. The fellow in this last episode seemed to wonder what the limit of that power was and whether there _was_ a limit to it. So, by becoming an Asura Cryin’, you have a _lot_ more power apparently.

As to why the folks in this episode wanted that, it appears that they needed it to prepare and/or use the splitter (or maybe to just stop anyone trying to stop them – they weren’t terribly clear about it). In any case, they were looking to gain the power to remove a burial doll from an Asura Machina – in particular, they were trying to remove their teacher from the Asura Machina that they had. So, they created the splitter to do it, but it was unlikely to work (hence why they needed a luck eater to use their luck to adjust the probability of it working) and it required the sacrifice of a lot of demons so that they’d have enough power to run it. Presumably it was the sacrifice of the demons why the teacher didn’t want them to free her from the Asura Machina and why she was talking to Misao to get her and her friends to stop them. Again, exactly how the Asura Cryin’ helped with this is unclear. But they needed a lot of power to make all this go and the Asura Cryin’ has more power than a simple Asura Machina, so that relates somehow. In either case, he likely made the pact with Nia’s sister so that he could use her familiar to hunt the demons that they needed for power to start up the splitter. So, even if being an Asura Cryin’ wasn’t necessary, it was a side effect of needing to make a pact with a demon and already being a handler.

In any case, it’s definitely not all clear, but it’s generally getting clearer with each episode. Personally, my take on it at this point is that the handlers are too anti-demon and that they’ve not given an adequate reason as to why Asura Cryin’s are bad, but presumably that will be clearer in due time. Of course, given that it’s the name of the show and that Tomo is now _living_ with Takatsuki – not to mention that he may require the power of an Asura Cryin’ at some point – I highly suspect that he’s going to become an Asura Cryin’ at some point here.

Wow. This is getting way too long. But hopefully it makes things more clear rather than less.

What the folks in this episode were trying to do [SPOILER] (and Tomo would love to do with Misao and Shuri would love to do with her sister) is separate the flesh and blood person from the mecha so that they can live as a normal human again. Apparently no one has found a very good way to do that yet however.