I'm interested in the statement "He's also pretty much guaranteed to get us a couple yards when the situation calls for it." I don't necessarily disagree, but it's one of those testable statements that might be inaccurate once we look at the numbers. So, last year, we ran with Asiata on 3rd and 1, 3rd and 2, or 3rd and 3 16 times. He got a first down (or touchdown) 11 of those 16 times.
Not guaranteed, but pretty successful.
Adding second down makes him successful 20/32 times (so he was 9/16 on 2nd down). A little less impressive to me, but I have no idea what the average back looks like.
For what it's worth, McKinnon was 1 for 1 on 3rd down and 6 for 6 on second down in the same situations.