frankly, never. he's a violation of all ethics. if you are religious, you might call him an abomination. conceived in an unethical way, a freak accident of malfunctioning technology, at the cost of two lives that are valued by many, connected to relatives or other people who love them. if there is someone to blame for attempted murder, it's tuvix.When is Tuvix allowed to live knowing he won't at some point be sacrificed "for the greater good"?
you can call him a sentient being full of compassion, but it's not the same as to conceive a child the old-fashioned way, bring it up, care for it, love it, and to invest many years and much effort into his or her development. to admit live conceived in ways like this is in the end the door opener for all sorts of manipulations, the next step is another augment problem or worse.