I guess this is a Miracle Max situation. They were mostly dead, whatever that means. The fact that they CAN be brought back means they are not completely dead, by definition. Dead is dead--no return.
It's pretty much at the crux of the ethics issue though, and if I recall correctly, this was never made into any kind of an thing within in the episode itself - that tuvok and neelix were somehow "sleeping" within Tuvix. Had the episode utilised that, I would not make such a strong ethical argument(on legal grounds it's still abundantly clear either way of course).
Without that factor, it's pretty much the same as getting them back by casting a magical spell that kills Tuvix, so I remain of the view that doing nothing was the correct ethical decision.