Long-haul in a narrow body isn't new by any means, but it's looking like it may have a resurgence with the A321XLR. I will certainly try to avoid it as much as possible though, as I generally find wide bodies to be more comfortable even if the stated seat measurements are the same. On a 4 hour flight I usually don't get up at all, but on a longer flight where I'm going to need to get up I certainly want to be able to move around for a minute or two and have access to the lav. One aisle means you're often blocked by the cart.