1st thought- someone posted about one of these things (voltage converter in USB plug) here recently and ran some tests. I don't think it had a hirose-terminated cable.
2nd- will it provide sufficient current? No specs at the ebay link for that.
3rd- it's a switching converter, and probably doesn't provide much if any filtering. Will a "dirty" supply voltage matter? Dunno. Probably depends on what it is feeding.
4th- not as efficient as powering with a battery providing the correct voltage, there will be some conversion loss.
5th- maybe nice to keep on hand for emergency use, to leverage USB "phone" batteries.