I've been working as a freelancer for many years, billing hourly. I have absolutely no hesitation to take vacations. If anything, it makes me more likely to take a few more days off, since I don't feel bad about a client paying me for that time. I routinely take months-long sabbaticals between gigs, and use them to learn new things. I take extra long vacations over the winter holidays.

If someone is feeling an urgency to work more hours beyond what provides true value for their life, the problem isn't how one's invoices are arranged. The problem is one's attitude.