My current company does this. In addition balances have to be used by the end of the year. People are encouraged to take their vacation, so this leads to a lot of people taking random days in December off. As a result you can't get a damn thing done in December.

I can't help but think that a system where accrued days expire after a certain amount of time or you can't carry more than a certain balance would mean that people's vacation time would be spread out more. As a result I could actually get some stuff done now that requires coordinating with people. I'm not sure if there would be a cost, however, in not being able to get stuff done at other times of the year.