Playing the devil's advocate here but I believe humans do not need preserving. With 7 billion individuals and counting, it seems we're doing just fine. Regardless of how poor the living conditions are for many of us.
For this particular species of insect however, having the last 24 remaining individuals of the entire species (presumably) be dependent on a single small shrubbery makes me think the effort put into this rescue was more than warranted.
Specially considering their extinction was (indirectly) our fault and a mistake that could have been prevented with better education and oversight on who and what was brought along during our travels. Had their extinction been an entirely natural process, I might have agreed that their rescue would not have been such a high priority.
Of course there is the question of us disturbing ecological balance somewhere is natural or not. We are after all natural beings ourselves. Whether we are intelligent or not and have the capacity to reason about our actions is irrelevant in a strictly biological sense. But this discussion opens up a whole different can of worms.
edit: Fixed some grammar.