it is not beneficial, it just makes stronger all the attacks with rend, as rend becomes harder to be saved by re-rolls.
The only circumstance that I can think of, that is a benefit for the defender, is when you want to re-roll of a save that (after modifier) will be successful, but doesn't trigger special abilities.
Example: Elven eternal guards can have a +3 to ST, and they can be buffed with the spell that inflicts a mortal wounds for each 6+. Against attacks with no rend, I can easily see you re-roll 3s and 2s, even if they would be successful saves.