@BlackYps Fair point, I expect I could get a much better variety in 1v1 with lower APM if I forced myself to play more relaxed and use weaker strategies (by virtue of my rating dropping giving me weaker on average opponents). That said though I'm naturally competitive so find it hard to avoid trying to play as best as I can to win.
@archsimkat Re your point on variety being due to a lower level of play on Astro, while I acknowledge ladder has a far higher skill ceiling than astro, the variety difference isn't simply due to a lower level of play on astro. E.g. For astro if I want a reasonable prospect of winning I'll need to scout my enemy regularly, micro key battles, determine my opponents vulnerabilities and exploit them, and determine how to counter my opponents strategies; by contrast 1v1 ladder most games around my rank my opponent will be doing t1 land spam, and I can do reasonably well initially if I just do t1 land spam (and am likely to do poorly if I don't do t1 land spam), with my strategic options reduced more to thinking where do I send my com and where do I try and raid and/or send my land spam based on where their land spam is.
Re the global ranking, your post actually highlights the value of the ranking system. If someone is consistently winning with a strategy (in this case having a teammate who gifts you your base on astro) then their ranking rises to indicate the strategy/strategies they use are likely to win. People will naturally look at ranking and expect the opposing team to in aggregate have a similar ranking to balance out, and the better the opposing team the less likely such a tactic is to work. People also generally join a game to play it, not gift their stuff to their teammate, so it's likely such a player will play with the same person for alot of those games making that other person's rank also increase (in turn requiring a better opposing team to avoid an unstacked game).
If by contrast you remove the ranking, that 2300 ranked player will appear the same as a low ranked player, allowing them to easily stack the game in their favour, resulting in far more unbalanced games and alot of inconvenience for people that want to play non-stacked games (who now have to resort to outside analytical options on each person that joins the game to figure out how good they are, and if you join the game as the last person or just dont want to spend ages ahead of any game researching your opponents, you're far less likely to realise you're about to waste your time on a stacked game against you). As arma mentions, people don't want to play stacked games, and on average you'll get fewer stacked games with the rating system than without.
E.g. glancing at a few replays of the highest rated astro player I could find (Harper, about 2.2k), in those games with good game quality (90%+) the games didn't appear stacked and Harper won some and lost some. If anything you're at a disadvantage at such a high rank, as there's not much difference between a 1600 player and a 2200 player on astro due to the lower skill ceiling, but there's more of a difference between a 600 rated player and a 1200 rated player.
Contrast this with the few replays I could find with a <50% game quality (there were far less of them), the team of higher ranked players crushed the team of lower ranked players.
All this is showing that the ranking system is a better than nothing guide to avoiding non-stacked games (even if it's not perfect), and also demonstrably not pretty useless (if most low GQ games are won by the high ranked team, and most high GQ games played by someone have them winning some and losing some, I'd say that's useful not useless). Most people don't like games being stacked against them, and I'd also consider it a bad new player experience.