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I do wish the had a "delete all" option, but I imagine that there's negative repercussions if someone uses it and then says "Oops!" and wants the mail back. Apple seems to have opted for the safer mode of requiring users to do individual "by choice" deletion of each email to avoid such accidental deletion of important documents.
With iOS 8, deletion of single emails easier. A single swipe right to left will send an email to the Trash. It no longer require selection, then tap. A single step process instead of a two step. That makes it much simpler.
I usually have my email accounts echoed on my iMac. I handle dumping large numbers of spam du jour email on the iMac where it is easy to group, select, mark as spam, and then send to Nevernever Land. With the ease of iCloud syncing, anything done to the email on the IMac will be mirrored on all my iOS devices.