It is helpful only in one situation - you are mailing stuff to your alt.
Ijn most times, i would want to either write a thanks letter to the sender, or, in case my kinmate sent me item for me to place in wardrobe and return, i prefer to be able to send it back, and non-deleted mails allows me to reply without writing down who sent what.
Now, if there would be third button made - get all stuff and destroy message- that may be a solution for one particular case, bit i strongly prefer that current way is kept as well.
Thank you, Turbine, for listening and giving us an opt-out of FE! Good work!