Even though being a scholar and good teacher may overlap I am not sure they are commensurates. If they were, we could come up with a measure of law teacher performance that accounts for both. I don't have a good label for that like economists have for all things good -- utility. But let's say it is called "condex" (as in contribution + index) and each faculty member would have a condex number based on scholarship and teaching. (I leave out service because at many schools service is directly related to what the dean asks you to do. If you are among the "ins" or really annoying, the dean appoints you and then awards you for doing what ever it was.)
So everyone has a condex number and there is a faculty average condex. This would nice because we could then get into the process of raising the average by working at the margin. By margin here I mean any change. For example if the average height of a basketball team is 6' 5" we raise the average by adding a player who is taller than the average. The new player is at the margin.
In faculty tenure decisions, the only way to raise the average condex is to tenure people who are above the current average. In effect, this means saying no to people who are better than nearly half of the existing faculty. It's hard to do but existing tenured faculty who are unwilling to do it (because, let's say and as I have heard, it would upset things socially) are shirkers. In fact, a claim that someone should be tenured because he or she did more that an already tenured person -- or even half of the tenured faculty -- is a non starter and a sure way to stagnate a faculty. Existing tenured faculty, above or below average, are sunk costs.
But what if a school really has aspirations? Does the average/marginal analysis work then? Going back to the basketball analogy. Suppose the team average is 6'5" but your goal is to have a team that averages 6'8". Now the strategy changes and the players in the 6'5" to 6'8" range should be skipped. Sure, adding a player who is 6'7" gets you closer to where you want to go but at the same time increases the size of the base you are attempting to improve on and uses up a roster spot for a 6'11" player.
So too with law faculties. A faculty dedicated to improving itself should understand and be willing to reject people who are actually better than half or even most of the incumbents. Adding someone near the average does little and actually hurts in the long run.