As with most of my training philosophies, I've learned the importance of hill work first hand, personally in races. Two corresponding examples are the Bear 100 2010 and 2011.
In the Bear 2010 (my first 100) I trained for the descents, especially the 3,000+ ft elevator shaft drop at mile 95 to the finish. I ran HARD down Flagstaff Rd. in Boulder 1-2 times per week leading up to that race. It paid off when I basically launched into a sprint on that final descent and finished 9th overall. The following year I was run-down from racing four 100s leading up to Bear and went into it a little cocky, thinking: 1. I had run it already and knew what I was doing, and 2. I had just finished 4 100s (San Diego, Hardrock, Grand Mesa, and Leadville, all within two months), so of course I was already prepared, right? Wrong.
My quads were blown out at mile 50 and I walked 80% of the remaining 50 miles throughout the night to finish one of my worse races to date. That last long descent to the finish was excruciating. I tip-toed sideways, backwards every step (maybe even crawled a little) with a dozen people passing me over the last 5 miles.
When I prescribe hill work to the athletes I coach, it's partially to get them stronger at climbing and/or stretch the LT but mostly to encourage the pounding of the quads on the descents. Next to fueling, I believe unseasoned quads destroy mountain ultras faster than anything. With blown quads you can't climb well and worse you can't descend almost at all, and eventually you can't walk.
When I got a glimpse at the Zion 100 course profile and saw the 2,200 ft drop to the finish, I knew I'd be prepared. The last several weeks have been 15,000+ feet of climb and descent. Last week was 17,050 ft climb and descent. I'm continuing that trend through this and next week and then tapering the vertical climb while building a little 45-60 min speed up until the final 9 day taper. Today's quad pounder was 15 miles, 3,226 ft climb and descent broken down over three sets of ~1,000 ft. Here's the elevation profile with the descents run hard, working both efficiency in short, controlled strides and pounding, long, plyo-bounding strides.
You can slog up climbs over and over during almost any race. You trash your quads early or if they aren't "seasoned" thoroughly, then your race is virtually and often literally, over.