Earlier quoted context omitted.
"doesn't mean they're unreliable" Well, from the point of view of an employer, it kind of does. They never know when you will leave for a new job.
I disagree - if the hoppers are hopping because they are not getting the professional growth they need, or that they think they're being taken advantage of, then that's highly predictable for you as an employer. Are you treating your employees fairly? Do they feel like they're contributing, or are they writing dead-end code? You know when they will leave for a new job - when you start treating them like trash.
1. The best people will quit.
2. The median people will whine and make the office a miserable place.
3. The worst people will put their heads down, terrified that they will get fired or laid off.
Consequence? The best people still in their chairs will be the ones with the lowest morale.