Since some attack powers seem to be able to replace basic attacks (note that magic missile and eldritch blast count as basic ranged attacks), the warlord may be getting a damage boost at 21st level via a boost to his allies' powers.
A warlord emphasizing INT and/or CHA over STR will probably love this ability, since A) the bonus damage will be higher, and B) the warlord's STR will likely be lower than that of the fighter type in the party, so the ally's attack bonus and damage bonus will be higher. Similarly, it allows a warlord to potentially do Greatsword damage (from his friend) while still getting the benefits from a shield or other less damaging weapon (say, with reach).
Is it universally useful? No. But as long as there's someone in the group with a better basic attack than the warlord, then it will probably come in handy.