Winston
In my experience with Japanese planes (which isn't a lot), I found that the hollow makes it much quicker to true the sole.

When I got my first kanna, I flattened the bottom with sandpaper on granite, and it took a long time. When I had to re-flatten it due to seasonal movement, it took a long time to flatten it again. That's when I realized that if I hollowed the appropriate section, it would be much quicker to true the sole, because only a tiny amount of material (at the toe and in front of the mouth) would have to be removed each time.

Now that I've thought about what David and Jim have said about it, I think that another possible reason that kanna have to be tuned frequently is because the small contact points result in faster wear of the sole, because the downward pressure is concentrated into those small areas.

An aside: I stopped using the sandpaper method because it was still kind of slow and inconvenient with my shop setup. I don't have a Japanese scraper plane, which is the traditional tool to true the sole. What I do now is, when the sole needs significant truing, is to use a metal jointer plane on the kanna's bottom, until the toe and area in front of the mouth are co-planar. Then I use another plane across the grain to make the hollow. Much faster than the sandpaper method, but of course it requires another plane, one that is well-tuned.