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.
In fact, if that was the way that the power transfer worked, Lilith could have just given Jacob the powers too and had him as a guardian in training. Instead, she gives him her powers and then dies, clearly intended to imply that once the power is transferred the prior host is human again.
