Well it's 9:56pm, Wed night, and I'm finally getting to this, after a busy day. I did get time off to run, run the kids around, karate, so it hasn't been a really long day, but a full one.
It seems that most people get a time off for time worked.
I think part of the idea of a salary is that they're not paying you for 40 hours every week. If so, then why not pay salary? The pay a bit more than you might make hourly with the expectation not that you'll work 50 hours every week, but if there are times that you have to, they have controlled costs.
The problems come when many managers don't manage your hours, but rather think the salary entitles them to work you as much as you're willing to work.