I agree that this is a major flaw with mail in ios. I get about a hundred msgs a day, most of which are handled on my iMac. But all mail also goes to my iPad and iPhone. Deleting all mail on both devices is tedious and a total waste of time.
One should be able to "Select all mail", unselect any to keep, and trash the rest. I'd think someone at Apple would have thought of this most obvious feature a long time ago.