With a single-threaded rails server, logging only the process id works fine. However with a multithreaded Rails application (Rainbows! for example) logging only the pid interleaves request output in an unparsable way. If it was an option, or the default, to log some combination of pid and thread id, that would allow parsers to sort out the log file.
I believe this was solved on commit 7e16fb9 😄
Fixed in 7e16fb9 as mentioned by @jmonteiro