Unicorn processes millions of requests per minutes for some very large companies. Changing the default actions of a core part of the web stack in a version bump could have terrible consequences for some use cases.
You're probably right that this is ideal default configuration, and maybe in the future it will be defaulted to on.
Right now though the best thing would get people who experience this type of problem to try the configuration change and report their findings.
I don't disagree at all with the decision to keep the standard behavior as default for now. For something as critical as an HTTP application server you definitely don't want any surprises.
I just wanted to get your thoughts on the tradeoffs and what you think is appropriate for the majority of use-cases.
If, under load, you can gracefully return an error and your user will just come back later (like the twitter fail whale), then you might as well keep your queue lengths short which close the window where a waiting user will hit refresh.
If you have some requirement that prevents the above, and have large queue lengths, then this may be very useful for you.
As well, some server configurations might not like getting the first part of the HTTP response much before the rest of it. Nginx will just buffer it all before returning the entire response to the client so it doesn't affect packetization of the response sent back to the end-user.
You're probably right that this is ideal default configuration, and maybe in the future it will be defaulted to on.
Right now though the best thing would get people who experience this type of problem to try the configuration change and report their findings.