Store emails with rendered variables

When sending emails with variables, these variables are not stored in the database. So it's not possible to control whether the variables were inserted or if civicrm has once again forgotten to make use of i.e. custom variables. Also if names change (marriage…) by now it's not possible to track which email has been sent correctly.

Example: /civicrm/activity/view?atype=3&action=view&reset=1&id=XXXX&cid=YYYY
"bla bla bla

bla bla"

Should be
"bla bla bla some_custom_data_as_it_has_been_sent_via_email bla bla"