yttriumMagikarpI don't know about that completely, I've seen some SC2 matches with maps that have 4 spawning locations and it has had an effect:
https://www.youtube.com/watch?v=Da66ACO5MyI
If it were fixed spawns, MC would've known where to 4gate with no problems. That was in 2013 though, but the latest was in 2015 at Blizzcon where they played Iron Fortress LE: https://www.youtube.com/watch?v=P9T_tVQdMoA
Which is apparently a a 4 spawn position map so idk. But don't the players themselves choose the maps from the ladder rotation? I definitely agree though that most of the maps played at the competitive level are 2 spawn position maps though.
4 spawn maps exist but they usually force cross positions. The rule of thumb is that the map must be symmetrical no matter what spawns occur. An interesting map is called Korhal Floating Island, where the map would always be symmetrical but depending on which set of spawns occur, it could play totally differently. This randomness never puts a player at an advantage or disadvantage though, because the map is always symmetrical.
The map you showed didn't force cross positions, which isn't always a problem except that the map was rotationally symmetrical, which isn't an okay combination. This doesn't happen anymore.
Maps are picked from the ladder pool, yes. Those maps aren't allowed in the ladder anymore though.
Oh my god I remember Korhal Floating Island rofl. Thanks for the explanation!