There's a strong correlation between speed and success as a CB in the NFL. Using 40-yard dash time, the critical drop-off comes at the 4.40 time. Jenkins has optimal speed for a CB with a 4.39 40-yard dash time. Any additional speed adds very little value.
Jenkins has also shown to be very smart and with a great understanding of defenses, illustrated by his "knack" for finding holes in spite of his athletic limitations.
An additional point is that he is great at catching the ball with his hands, something that would prove especially useful given the potential change in position.
It's true that he isn't terribly physical, needs to get stronger, has limited jumping ability, and his effort has been questioned. Still, given our need at CB I wonder if he'd be a potential convert... something Harbaugh has already shown a willingness to try.
Update: This isn't a condemnation of AJ Jenkins ability. It's simply a thought-experiment to kill time before the divisional round.