No, not the shortest, but the most fun and the most scenic. The most likely to have a place to pull over and be alone in the desert. The most likely to see cactus in bloom. The best place to take an RV and test it out. A really good rock collecting place.
We absolutely hate the I-15 (well, I do and I do most of the driving) and I've been that way a few times. I love the I-40. So going through Kelso and seeing the Preserve is way to go without semi trucks and a million cars - for at least part of it.
Going to Vegas is not a major destination for a lot of people in the Colorado River region - it's a good place to go out for dinner and stay in air conditioning (for cheap) while exploring the desert. We stop by Vegas on our way home from GC, even though it's out of the way.