lucy24 - 12:09 am on Aug 11, 2013 (gmt 0)
my wife and I -- both sitting in our home office -- would show up with the same IP and different UA's
Yes, that's the drawback. There are rare situations where one IP : multiple UAs could be a legitimate human activity, like you turning to your wife and saying "Check out this site, here's the URL". I mostly get it for unfinished ebooks, where someone's checking whether it looks the same in MSIE and another browser.
You said somewhere along the line that the follow-up requests can come up to an hour later, right? So you can't even say "If these two requests come more than five seconds apart, it's two humans and I won't worry about it." You could block obvious robots, but you'll be left with a lot of false negatives.