If one is going to argue that Tuvix wasn't a lifeform, I think a serious blow is being dealt to any argument that Data, Moriarty, The Doctor, and possibly any unplanned children have any right to self-determination.
If a mother died giving birth to a child she didn't want to have or that wasn't planned, and she could be healed by killing the baby (organ transplant, let's say...it's a hypothetical scenario), would it be "right" to kill the baby to restore the mother to life?
If I were Tuvix and was told that at any instant I might be summarily executed to restore to life two people who I was arguably best qualified to state would not wish to be restored at the expense of my own life (and how I wish the episode had shown -that-), I would likely ask to be placed in stasis. If I'm a lab experiment, treat me like one. To allow me the illusion of life with no true freedoms is cruel not just to me but to anyone who places any value upon my existence.
I can't imagine that things would have been so cut-and-dried for Our Heroes if they'd been in contact with the Federation, and it scares me to think that Janeway took this level of Authority upon herself without even the pretense of a formal hearing.