OK, I'm running through what happens if the Ravens kick the FG. I'm assuming the Bills drive pretty much unfolds similar to how it actually did because that's the data point we have, though butterfly effect and all that.
Bills get down near the goal line, still down 3--so while they'd like to use up clock, the need to score comes first. I think it's likely the Bills end that drive with a TD (not guaranteed), though maybe they consider kicking the FG to tie if it stalls. But either option occurs with more time on the clock because you cannot kneel and run the clock that way.
Best case scenario for the Ravens: they hold the Bills to 3, Jackson gets the ball back with about a minute and 0-1 timeouts, needing a FG from the league's best kicker to win (OT if they can't get it)
Worst case scenario: They give up the TD, trail by 4, Lamar has about a minute with 0-1 timeouts to go 75 yards and get a TD.
That worst case scenario basically boils down to: you need a TD. If given the choice between 1 play from the 3 yard line, or having to drive 75 yards in a minute possibly without timeouts, I'm choosing the first option every time.
Also, in the Dolphins game, the Ravens did kick a FG and it didn't work. I'm sure that was on Harbaugh's mind as well.