desmond's "power" was to withstand crazy power things. jack's resistance was that he was the new jacob at that point.
Jack give up the Jacob position and made Hurley the island's protector before going into the cave.
So, either you can have multiple guardians all at once (which was never once even hinted at and, in fact, was more or less contradicted by the fact that Jacob and his mother kept blathering on about a successor, not successors) or Jack was as human as MiB when he went in.
