What is PTO?
Usually, the combination results in fewer days of available leave, and since vacations are often planned months in advance, people are forced to come in to the office while sick or spoil their entire vacation by screwing up the itinerary.
It's pretty much inevitable that pooling sick time and vacation time results in a lower total. People aren't really expected to use all their sick time in a given year if ever. But for those who do plan to maximize their use of vacation, it means there's not a lot of margin when they do get sick and it encourages behaviors that aren't good for anyone.