Too many exceptions ........ Might as well enter and exit positions before 4:00 PM EST and don't get caught with your pants down.
Holding to expiration is an edge case that has interest to some of us but you're right, probably not to most. There are a strategies that take advantage of the fact that a cash settled option doesn't cost you anything to settle, no bid/ask spread or commission. And the bid/ask spread at the end of the day is pretty crappy, often even the mids are outside the max intrinsic value of a spread, for example you have to pay $5.20 to get out of a 5 point spread.
All the SPX options trade until 3:15 pm ct. The PM settled options on expiration day, stop trading at 3 pm ct because they settle at the close. The am settled options stop trading the day before at 3:15 pm ct, but settle based on the friday morning opening.
Um, yes, I think we established that. The question was why the difference between the pm settled SPX and the pm settled OEX and XEO.