I think IGO side already had a tremendous loss from NEO. If Aimaru would be a NEO member, that would be too much. I think we've already seen all NEO members from IGO side. What's left to be revealed are those from the Bishokukai.
Well, I agree, we've already seen too many IGO defected members. Now we need to see the members from Bishokukai side. It seems for me that Joe Joe will be among them.
And well, the mysterious person who attacked Teppei really used something like Aimaru's move. The thing is, I doubt it is excusive for Aimaru. I believe that at least the likes of Setsuno, and Acacia's members know about this move and can use it propperly. So this mysterious person might be able to use it as well.