I don't think that is a requirement for your hubs to be listed by search engines. I just joined yesterday. My author score is still lower than 75. My first and only one hub has already been indexed by Google and I have even received few referrals from Google today. The hub score and author score are just internal ranking system used by HubPages.com. Search engines don't recognize them. They have their own algorithm to rank the search result for each keyword.
I think you're right. My hub scores don't affect my traffic either. Some of my low-score hubs get pretty good traffic actually. But still, I like high hub scores; they make me smile :-)