Rava asked, since they are all immersing in a Mikva (in addition to taking the shower) than there is no damage being done. But why is there no concern that perhaps occasionally they will neglect the Mikva altogether, since they have the erroneous perception that the shower is the primary purifier?
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That is precisely how rava explains the concern; (they will think only the shower is needed as opposed to abayei who said they will think both are needed)