I don't know if that's what people mean when they say second brain? My "second brain" is basically a memory expansion/thought recorder. With the understanding that the memory isn't truly integrated like regular memories, which has plusses and minuses. A lot of value for me is just getting an idea out of my head if it's in the way, even if I never revisit it. Helpful for adhd and otherwise distracted minds as long the process of recording is very cheap. Which, as you say, yields mostly garbage.

Where it works better is as a place to store more careful thoughts, and as a place to force you to express those thoughts in writing.

That is, it works best if you don't try to use it as a literal second brain, don't rely on it doing any thinking for you.

All that being said, confession time: using obsidian for 6 months as second brain among other things, i don't believe it has really helped me think other than as being a place where I also create more polished notes where I must crystallize ideas.

What it has done is enable my idea hoarding/belief there's gold in them there old thoughts, so I spend too much time checking if I already had a thought rather than just thinking. So the verdict is still out for me.