RD introduces some neat things mappers can use for their unusual game modes. I think that's why you see the mode being used for these non-RD games.
Asteroid is a bit confusing for new players, but I don't think it's the scoring system that troubles them. I think it's coming to terms with the requirements of the map and the game mode. A team has four things to do: 1. Protect their core, 2. Protect their robots, 3. Attack the enemy robots, 4. Steal the enemy core. If you play any stock map on Valve official servers, you might think that a lot of players are less competent than bots. When these players only have a single objective, they can do some good simply by wandering over to it and rubbing up against a capture trigger for a few seconds. Imagine how long it must take for them to completely comprehend Asteroid.
The scoring in Asteroid is straight forward. You kill enemy robots and collect their cores. As you kill one set of robots, the next set is unlocked and you must travel deeper in to the enemy base in order to reach them. 'A' bots drop the lowest amount of cores, while the 'C' bots drop the highest. At any time, you can venture to the back of the enemy base and steal their reactor core. This is a flag which is a type of container. The longer you stand on the point, the more the container is filled with robot cores. You carry the enemy reactor core back to yours, as you would with intelligence.
When a team reaches the win limit of 300 cores, the game starts a countdown. At the end of the countdown, if the team still has 300 cores, they will win. This countdown gives the enemy team chance to make a mad push for the core, to delay or prevent the victory by stealing it.
The water drop-down hatch, just inside the front of the enemy base, gives potential core stealers an often overlooked, and discrete path to the enemy core. But when you try to escape with the core, you can't take the same route back, so it's a bit like a mouse trap. Thankfully there are plenty of places to hide in the base.