My entire career has been under Use it or Lose it. I would love to have a cash out option, even at a reduced rate. I get 3 weeks, but cannot possibly take 3 weeks. Nobody covers for me, and the work would simply pile up to unmanagable levels. No spring chicken, I cannot work to midnight etc. anymore.
I think under the "billable hours to customers" business type, this kind of thing would really be key. It'd be worth more to the company to have you on duty, so they'd be willing to cash you out.
I'd like to see more equity in those key days/weeks, such as Christmas week. The same people keep getting those choice slots, year after year. I always work them (even the day after Thanksgiving). Getting a little sick of that.