because if you HAVE to change your flight and there aren't any I-class seats available, you're seriously pooched on getting a confirmed J seat. That confirmation is worth it's weight in gold, imo.

When I was doing pre-sales consulting to Asia, we never bought full Y fares - it was usually a small jump up to a biz fare and we were guaranteed a seat in biz (with a shot at an u/g into F with CX or UA).