Wow - some insensitive and unthinking responses here.
I scan through my emails on the phone when away from the desk/pc,etc. Those that need a longer response, or more indepth reading will be handled later when at my PC. So, some (not all) emails do get read twice. Waste of time? In some cases, probably. Not reading my email for a day can also be a problem. To each their own.
If somebody wants to do that, why not?
Meanwhile, along with lots of other solutions, why bother to delete it. You can set the iphone to only store a certain number of messages, and it will NOT store thousands. Just one solution.