> I've talked to some other professionals about this, and for many of them, they readily admit that they feel like they need to be at work for their employer to function properly. They also admit this is their own impression, not something their boss has given them ("if you take that vacation, I don't know what we'll do without you").

I believe there is an ego component to this. Not only does it "look good" to upper management, it makes the individual feel like they are indispensable to the company. Only the unimportant people can afford to take vacations.