If you're going for most logical cause, then 'not getting out of a threat stance when you're a threat-statted tank switching with an evasion-statted tank' seems to be the most logical cause.
So, let's look at how that discussion would go:
Tank 1: Hey, I'm not generating as much threat as I should be. I now, others that play the same class don't really have this problem, but there's nothing I can do to fix it, so, I need you to switch out of tanking presence when you aren't the active tank.
Tank 2: So, because you're bad I have to be better? And we let you tank why?
Ignoring accuracy doesn't work anymore. Whether you want to emphasize it or not is your call, but you cannot ignore it, and doing so is simply a sign of ignorance. It doesn't matter if that is the style somebody chooses to go with, what's wrong is wrong. Ignoring accuracy leads to increased damage intake and reduced threat output. A tank has two functions: staying alive and holding threat. As such, ignoring accuracy is directly counter to fulfilling a tank's purpose.
Your entire argument is that if the OPs stat strategy is working for them then they should keep with it, which is fine, except that if the OPs stat strategy was working for them this post would not exist.