Going away to do a startup doesn't send the same signal. I'd gladly hire someone who did that.
You're wrong. I can make the same argument against people with long tenures. Some people, when they realize that they've been passed over for a promotion or that they're not being groomed for the role they want in the future, bounce (external promotion). That's the honest thing to do. Others stick around, play politics, work the system, slack, and wait for others to fail so they can capitalize on the chaos and move up the ranks. In other words, one could make the argument that the able, confident, ethical, honest people job hop and the political actors stick around and climb the ladder.
I don't actually believe that most people with long tenures are unethical. I'd put the correlation right around zero, to be honest about the whole thing. I think that there are patterns that cause good people to have long tenures and bad people to have short ones, and also patterns that cause good people to have short tenures and bad people to have long ones.
First, if this guy is really so great, why has he flamed out of 3 firms in 5 years?
Maybe he's technically excellent but bad at playing the politics. Perhaps he's an overperformer, like McNulty on The Wire. Maybe he's just unlucky. Maybe he didn't flame out, but each move was a major promotion that he wouldn't have been able to get internally.
Second, do I really want a guy who's bounced from firm-to-firm glomming up ideas and IP? Won't he do the same thing to me?
The problem isn't "job hoppers". It's attrition. When people leave, it's disruptive (and the internal disruption to your own processes is so much more of a threat than IP leakage, unless someone's actually stealing code and then it goes to the courts, that the latter is a rounding error). You can control attrition by treating people well and making sure that ambitious people have appropriate opportunities get promoted on time. It has little to do with the people you hire, and much more to do with how you manage them.
The anti-"job hopper" sentiment isn't really about IP. It's about being mean-spirited, and the IP justification is just made to back-fill an already-formed prejudice.
Not everyone who downvoted that (admittedly, not well-expressed, insofar as some thought I was accusing Spiegel of causing the Sony hack rather than simply benefitting from it) comment was a Speigeloid. The Spiegeloids are the ones who have been repeatedly downvoting random comments of mine just because I wrote them. (Or, an alternative explanation would be that the Hacker News community has suddenly become much meaner... but I doubt that.) In other words, stalker-downvoting (seeking out someone's comments and, without reading them, downvoting them).
I have no idea when it comes to their personal identities, but I know that I have at least 4 stalker-downvoters. They could be sock puppets of one person. For all I know, it could be Spiegel himself (but I doubt that).