What I mean is that with X, whenever you make a date change, the sector availability may not be there due to married segments logic. With O, if no space within 24 hours, I can make the stay just over 24 hours to get around the problem. Even ignoring married sector logic, it is certainly possible that individual sectors may not have D class on date I need. (Say IPC scl bog) .Ticket the scl as O allow more flexibility in date change.
It is especially bad on QR. One time, CX had to call QR to request the space (luckily, QR gave us the space). However, it makes the date change process more complicated.