That trend has always annoyed me, mainly because having both migraines and a job with one-pool PTO make it impossible to plan a vacation pretty much ever. These days I schedule my vacation time out of a pool of time that is just vacation; when I am sick, that time is compensated according to a baroque1 system largely defined by the Belgian federal government, the workings of which are mostly opaque to me apart from the bit where I have to get a doctor's note if I'm going to be out for more than two days. To give you some idea of how opaque I mean, I have no earthly idea under what circumstances they stop paying me if I'm sick2. We are talking about the Benelux, though, which when it comes to social welfare is basically "Scandinavia but less pushy about it."
Even in the US, though, I think there could be a lot of value, in terms of employee quality of life, in splitting PTO back up into vacation time and "emergency time". Being able to schedule time off for planned events (like family trips or conferences) is one kind of benefit, being able to take time off for unplanned emergencies (like car trouble, or needing to pick up a sick kid from school, or your basement flooding, or being sick yourself) is another kind of benefit, and not having to expend effort on budgeting under uncertainty between the two is yet another, albeit more indirect and less obvious. Still, less stress is less stress.
If I ever run a company with full-time employees again, I'm inclined to try this.
1That is to say, like everything else the Belgian government does.
2Although apparently I have the statutory right to take up to two weeks off in a row, if I have enough time banked to do so.