I think a union between them is a good thing. I think it will make it harder for Netanyahu to leave out Ben Gvir if he is tasked to form the next government. On the other hand, it might make it easier for Netanyahu to also leave out Smotritch.

But in general it is a good thing. 

They are arguing over the identity of the candidate that will take the 7th spot on the joint list. Maybe they should offer that to Avi Maoz of Noam, though it looks like they are going to leave him out completely. 

Unless they can convince him to drop out, probably by convincing Rav Tau that it is necessary, leaving Maoz/Noam out of the joint faction will end up wasting votes. I don't see enough votes for 3 extremist parties on the right,  with the Likud staying in the 34 range. Bringing him in to the joint faction will make it even harder for Netanyahu to bring this party into a future coalition. Netanyahu isnt into these extremists, despite his talk of a full right wing government, על מלא.

Despite it all, from a social perspective rather than a tactical perspective, I think we need fewer parties not more and that will be socially better (less election violence and fighting among people who are 99.9% on the same page). On so many of the issues they are similar enough that they should be running together.