I tried searching through all Splunk Answers and couldn't find a detailed explanation for this. I know you didn't want to just hear something from the documentation, but it does explicitly say "Required. Splunk will not start if an index lacks a valid thawedPath." So it doesn't seem like there's any ifs ands or buts with this setting. If I hear a more in depth explanation as to why it's required, I'll post an update here.