Changing membership type deletes previous contribution line-items

Seems odd that no-one has reported this, but working on CRM-15861 and go through a series of tests, noticed this scary bit in CRM_Member_BAO_Membership.create, ~ line 336:

    //do cleanup line  items if membership edit the Membership type.
    if (empty($ids['contribution']) && !empty($ids['membership'])) {
      CRM_Price_BAO_LineItem::deleteLineItems($ids['membership'], 'civicrm_membership');
    }

This seems particularly bad. Why would changing a membership type result in the deletion of any contribution line-items made in previous years?