I do believe IR gets worse over time in situations that don't show it or show it mildly.
My iPad 4 in the 'iPad 4 image retention' thread showed it slightly. On a family member's iPad 3, it was terrible after 5 minutes of letting the checkered board stay up on the screen.
On another family member's iPad 2, which was quite a bit older, showed IR in like 2 seconds (the search bar was burnt in against a grey background, and I literally had been using it for 2 seconds!)
But nobody in my family has complained. Must just be me and my OCD.
Still, this is most unacceptable on a laptop where windows sit open for a very long time sometimes.
Tablets too, imo. But I could see how that would slip past one's radar more easily.
I'd say no foul if they corrected the issue with LG's instead of continuing to pump them out with the issue, with caveat emptor after the return policy is up (to stay in a retail situation) or dealing with a out of warranty swap if you begin noticing it after the included warranty is up (assuming you didn't get AppleCare or have AmEx double warranty, but still a PITA).
Seems inherent on every iPad 4 I've come across thus far but some claim that they don't.
I can also easily observe the subtle differences in contrast and text crispness between LG and Samsung screens on MBA, even though it's a TN panel and does not have IR in this case.
Glad I have a great iPad 3 and a Samsung'ed 2011 Air.
Even if I had the means for a rMBP right now, not sure I'd want to be playing the swap game.
I swapped my iPhone 5 out quite a bit too for vertical interlacing. And one phone was horrendously unstable or so i thought but it was really just a nasty bug that got ironed out in 6.1 onward.
Why not move to Android?
I like Apple products so much I keep swapping until I get a good one, and I keep getting iOS products even though I hate them stock (after buying iPhone 5 at launch, even though I enjoy/enjoyed the 4" screen and LTE and new design, I will never buy a stock iOS device again. It greatly hinders my overall experience).
Call me a fanboy, not a hater. But most certainly a hater of their 'Anything Goes' Quality Control.