This post is backwards. What Holl does well, which is separating players from the puck on the rush and in the corners, he does a dozen times every game. Some speedy bastard like Suzuki will come flying through the neutral zone and attack our blueline, and Holl just calmly angles him into the boards, pins him, and shoves the puck into the corner where one of our forwards or his D partner retrieve it. His distribution is heavily weighted towards these average plays which he makes with no fanfare and no fuss, it's just his job and he does it well. It doesn't end up in the highlight reels because it's just fundamentals.
The reason that people write long posts about his catastrophic mistakes is that when he does screw up, such as a bad outlet pass or puck watching in front of the net, it's more likely to end up in our net. As you correctly identify, it's a catastrophe, rather than a mundane mistake. Your very next sentence implies that his distribution is towards average games where he does his job and then will have a single catastrophic game of "Justin Loll" (which is an excellent nickname and I will steal it).
Keefe throws Holl over the boards because he knows that 98/100 times Holl is going to do his job defensively, and that we have the offensive talent and defensive group to make up for those mistakes. Keefe literally plays him to his strengths and accepts the weaknesses, and that's why we're a top defensive team even with Holl playing 20 minutes a night.