Anyone know a reason why they erratta'd spellcasting foci so that they couldn't be used on drain resistance tests without changing the others? If not then I'm inclined to allow its use for drain resistance (in any games I run). I don't see any reason why it would be unbalancing or illogical or anything along those lines. If you have some idea, I'd appreciate hearing about it.
To be honest, I'm not even sure whether it was intentional, or simply sloppy editing.