Lucean, all the same arguments apply to weapon foci knucks directly. Why pay for killing hands when you can just use weapon foci knucks whenever you want?
Knucks are a restricted item and can and should be possibly detected and may therefore (depending on the place) not be available at every opportunity. So no, the arguments don't apply to weapon foci knucks.
@All4BigGuns
An argument that something is needed for non-Trolls should also include a sentiment about that same option being applied to Trolls.
But reviewing the Street Magic version from SR4 one could argue that it's just again an oversight, because the "older" version was applied to
unarmed combat attacks, so it seems like a reasonable house rule to modify the power to reference the used skill.
Since the attack still
gains AP of a certain value and doesn't
modify AP it would be of no use with Shock Gloves or implant weapons (depending on the level) so IMHO there still seems to be some balance.