A creationist might, in turn, argue that it makes no sense for humans to "keep" the appendix when it so easily gets inflamed and bursts -- surely natural selection should have ensured that it disappeared entirely? (It's rare in my experience for a creationist to know enough about evolution to construct such an argument but it's a hypothetical possibility.)
The simple answer is evolution doesn't have any "sense". Evolution doesn't make cognitive decisions. Evolution does not step in and cut off unnecessary organs in an instant.
In order for the human species to lose any organ, it would first require at least one individual to be born with a mutation that meant it did not have that organ. It would then require that individual to survive to reproduce and pass on the new gene. It must then be a significant survival advantage not to have that particular organ. It would then require sufficient time for that gene to spread across the population.
Before all those things happen (which they may never do), it is entirely logical that an adaptation may occur, such as in this case where it is used to store bacteria. There is no predetermined outcome for what then happens to that organ. It may eventually (possibly in
millions of years' time) be lost altogether, or it may remain for its existing adapted use, or it may adapt for another use entirely.
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