It's not always that simple. DH is not an hourly wage earner. He gets paid for days off, but he also has a minimum annual billable hours threshold; one that would be impossible to reach without working lots of nights and weekends if he chose to take long vacations. Most of the time, small business owners are in the same boat -- it isn't always a direct pay loss for those days; it can be counted in lost opportunity and revenue for those days as well. Also, DH can't easily just leave town -- if a client has a court date or has been subpoenaed, then he has to appear if he cannot find someone else willing to cover the court date, and his own plans don't matter.
I'm salaried and am fortunate enough to get quite a lot of paid vacation time, but I also have a young child at home, so I have to use 2-3 weeks of that paid time off dealing with the days when my kids are are not in school: usually 10-12 days at Xmas, three for Veterans Day, MLK's Birthday and President's Day, five to 7 days for spring break, two each semester for "faculty development days" and usually at least one week each at the beginning and end of the summer when daycamps are not in session, and then any days when they are ill but I am not. We overlap some of those days with travel, of course, but unless I travel alone with them, not all of them, and holiday-period prices are much higher than at other times, so it is often cost-prohibitive to travel then.
PS: Angie is mostly correct about billable hours, but it is often a bit more complex than just tying pay to how much one works on a given project. Many professional service firms in such professions also have an annual threshold, and keeping one's job often depends upon reaching it. Hours billed in excess of the threshold often mean bonus pay, but if you don't manage the minimum you will usually find yourself on thin ice. This can even apply to partners; an unproductive partner is still collecting a set share of the revenue while not necessarily equally contributing to creating it; and after awhile, the other partners may tire of carrying that kind of dead weight. In most firms, one's own pay normally washes out to between 25-50% of the revenues collected by the firm for your work or that of the people who work for you; the actual rate of pay depends on several factors. (The remainder of the revenue goes to cover things like support staff pay, office rent, and various other overhead costs, just as in any other business.) So, for example, if you bill out at $200 an hour, over the course of 2 days you personally might be losing around $1100 of gross pay, but the firm that you work for will be losing $2600 in revenue. That's a substantial chunk of change, and unless you have a very large family, the difference in domestic airfare and driving expenses will only account for a fraction of it.
Statistically speaking, flying is waaay safer.
