She did the correct thing, it was more selfish to let Tuvix live, he had no actual family or friends because he was neither Tuvok or Neelix, there's not one logical reason that he should have been left as is.
The tuvix debate is an offshoot of the the most important trek debate of all. “Who is the best captain?” We won’t let the fact that there is an obvious best captain get in the way of debating over it.