Not an accountant, but that was basically the same explanation given when my company went to no carry over of vacation. They did it in stages -- originally you could carry over the amount of vacation you earned in two years. So if you earned 5 weeks/year, you could carry over 10 weeks. They whittled that down to only carrying over 2 weeks, and then finally none. The only "buffer" we have these days is that at the beginning of the year you can elect to buy an additional week of vacation, but if you don't use all of that week, they pay you back for the unused portion in our last paycheck of the year.
Our company also went to unlimited sick time a few years back, and they gave the same reason -- carrying sick time on the books was a liability. Now it is basically up to our direct manager to watch that we are not abusing sick time. In general, it works well and I have never really seen anyone abuse it. A huge benefit is that people stay home more now when they are feeling sick, so they don't spread it to their office mates.
There are also a few companies that I know that have "unlimited" vacation time (no liability on the books), but friends that work at one of those all tend to agree that people end up using less vacation time, because there is internal pressure that looks down on those who take too much vacation, and those people tend to be ranked lower, get less raises, promotions, etc. At least at my company, we are always encouraged to take all of our earned vacation.
Kurt