I also don't understand the logic proposed both here and in the PC election thread for removing the global ranking system.
I think it makes sense that fun and casual game modes such as Astro crater, FFA, Phantom, etc. are unranked. Imagine those game modes actually contributed to your rating. People would exploit those game modes to gain rating! Imagine being able to host an All Welcome Astro crater lobby, put any 0 rated players (that is, the 1500±1500 player with no experience) on the other team, and have a teammate just give you his eco; if you are a half decent player, you would probably win almost every game and gain a large amount of rating while doing so.
Oh wait—that already happens. I saw this player with 2300 rating a while back, and I wondered who he was. Must have been a great player I somehow missed, so I went through his replays. Turns out, he had played Astro Crater all welcomes exclusively, and every single game had his ally dump him mass for a very fast all-in Czar rush. I don't even think that strategy is particularly good; if you know it's coming, and have 1 teammate give you his eco, it's pretty easy to counter. But he was able to get to 2.3k doing only that. Come to think of it, if I played a ton of all-welcome astro craters, manually stacked the lobbies, and had both my teammates walk at the enemy after giving me an initial set of buildings every game, I could probably get to 2.5k global or more.
It's just an easily exploitable system that decreases the integrity of global rating as a whole. In theory global is already pretty useless for balancing on astro or in general because of how easily exploitable it is, but thankfully right now most people aren't like this guy or Suzuji (who got 3k rating from playing Dual Gap all welcome). In my opinion it's probably better just to keep these unranked so people can actually play for fun, especially since they aren't being played very competitively anyways.