1. It is quite literally impossible to be more effective than both Tuvok and Neelix. Both had their own individual jobs that were now collated into a single being. He had less time and availability to answer all issues. He cannot be in two places at once.

2. If Tuvix was injured or killed then two roles were left unfilled.

3. While ‘sort-of-but-not-really’ the same person, that didn’t stop everyone from feeling the loss of Tuvok and Neelix. This new guy may have had the same memories and experiences but he had a completely different outlook on all of those memories. Relationships that had been built with both crewmen now were left to wither and die.

Tuvix was already damaging crew morale. There’s a reason that the bridge had zero response to him being led away. Tuvix wasn’t their friend and never was. He wasn’t the person they had built a relationship with. He was a perpetual reminder that everything they knew was gone. They lost not only their morale officer but a senior officer.

The needs of the many were fulfilled by unmaking the mistake of Tuvix.