It seems like Paul is only looking at one side of the table: the one to be hired. Certainty, we're all able to switch to better opportunities, but the long term signaling effect says you're not going to be their in tough times. Persistence is key in startups, so I can see why he'd be anxious - as would I.
I think loyalty is something earned. I think we should model jobs more like relationships, value has to be given on each side of the equation. If your not treating the girl well, she leaves. You don't go in saying, baby if you date me you have to be here for the next 2 years even if I start beating you.
I've heard similar advice about hiring people who are obviously over-qualified and will probably leave in 6 months. You have to use them not as workers, but as trainers and inspiration beacons for the rest of the team.
A plurality of companies have been signaling that they won't be there for the employee in tough times. Goose, gander?
EDIT: You mention specifically start-ups. Ok. There, however, there is reward presumably commensurate with the risk a participant takes. Also, the work is probably more challenging and interesting. I hold that neither particularly apply to other sectors, speaking generally.
There's a fine line between being tough and taking an unnecessary risk. If you're not in a position where you can walk into your job to find it doesn't exist anymore, it is your obligation to yourself and your family to manage that. If you can take the risks to stay, why couldn't you also take the risks to change jobs often?
If you're looking only for people who can put themselves in that position, then I'd bet you'd find a good number of them to be job-hoppers.
How tough are we talking about? If the company runs out of money and asks you to work for free for a few months, I wouldn't fault anybody for leaving. As others have said, working for someone is purely a business transaction. If you aren't getting what you want, you are free to go elsewhere.
actually, I'm looking at it from both sides. I'm working on my own startup right now and I hope that I'll be hiring people later this year. My point is that job hopping doesn't factor in as a signal on whether your resume ends up in the interview pile. I'll let you explain in the interview about your experience rather than just assuming that you're a disloyal sociopath that only looks out for themselves.