Mapgens are very well suited for low rated games. They are way less intimidating. The simple fact of the matter is that new players have repeatedly expressed their discomfort with having to learn maps as well as game mechanics at the same time, and even though sometimes they communicate that discomfort poorly, no amount of quibbling over what "build orders" actually are is gonna make their reluctance go away. If the goal is to get noobs to play ladder instead of custom games, they should be included more for that reason alone.
I'm not sure teaching players specific game mechanics should be the main goal with ladder map composition. Even though we call it ladder, the vast majority of people actually stop climbing it as soon as the rating system zeroes in on their skill level, and it should primarily be fun to play at your given level as opposed to a challenge to learn boat or transport micro or whatever, at least in my opinion.
Maybe once ladder/tmm games become prevalent, the focus could shift, but for now, my understanding is that custom lobbies still dominate.