The peoblem is not with players leaving their alliance to join another. The problem is that they shipjump, spy and take pnaps.
Leaving ur alliance after a round is fair enough.
Tell them thanks for a nice round, u stay friends with them, and hopefully u work with them again later. In whatever form.
I have been in 100000 alliances in my "career" pia and pa. I dont see that as a problem. I am Omen hardcore core now though..so I m stuck
damn u SB