That said, I don't see much point in taking all these scenarios in vacuum because they're highly unrealistic. They are a little black and white, comparing a complete pleb who has no clue of the LK fight, with someone who's well-prepared and anticipating fight mechanics.
Obviously Nitro Boots holds no benefit if you don't know what's coming at you, and don't even know where your keybind is.
It's just as obvious that Nitro Boots are exceptionally good if you know what you're doing.
I invite you to take John Doe as your raider example. He's your average Icecrown raider, knowing most of the tactics in a dulled-down way, but is still a bit clumsy at everything. He knows the tactic is "Run from Defile", but is too caught up with his rotation to look at timers or plan ahead as to where he's going to place it.
When his DBM yells at him, John will quickly hit his panic button and try to run away, putting his defile way farther than he would have, had he not used Nitro Boots. Most of the time this means - Crisis averted.
I'd say this is a more realistic scenario to consider, rather than just saying "A good player doesn't fail". Most of the people fall into that grey area between absolute pleb and hardcore raiding machine anyway.
Though I'm quite sure everyone has lapses in concentration, so I stick by my first statement in this thread.
For the sake of the argument, try to tune in to various streams for a week or something. Watch different guilds' runs and try to see how many wipes could've been prevented if the person causing it had Nitro Boots. Also try to see how many attempts would have been a wipe if the Engineer did not have Nitro Boots.