If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
No, the main subreddit is weird and a lot of things (eg, tags) that don't specify a subreddit are in main. The All subreddit ought to do both main and discussion, but its tags only cover main. Also, there are a lot of bugs in main and all.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.