It's even more not worthless if someone uses their healing power at the end of an extended rest and the gm allows the to stack. (You ARE spending a healing surge at the end of an extended rest to regain hp, just you are using a power to do so...)
Because using that ability doesn't take extra resting time? And because the bard's Charisma bonus is likely to be higher than the statistical average of a d6?
Except that it's not.
1) The PHB itself explicitly calls this out as a valid example of using encounter powers between encounters, as Smeelbo said.
2) This question has been asked roughly a thousand times since 4E came out. Customer Service answers and posts by the development team have all said that not only is the ability to use healing word outside combat within the realm of the rules, it was anticipated and expected by the design team.
The Bard power is not worthless because it doesn't require repeated rests.
Allowing use of encounter powers during a rest is not broken at all.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.