I've certainly been delayed on long haul by 12 hours for a u/s aircraft. As is pointed out above, the difference is usually the ability to source a spare. In this case the spare was found but it went u/s as well! Finally crewed into the original jet, got told it was still u/s, got off, got it fixed and got back on again. A right kefuffle, but sadly it was the Newark departure lounge, not a 5 star hotel. 15USD for food (thanks United) and 400USD voucher compensation for arriving on a different day to the one I was meant to!