I've been shooting PRS for 5 years and have never seen a malfunction caused by a chamber flag. Nor has my chamber been hot enough to melt plastic. Especially not my freaking 22.
But I have seen a couple of guys with mag blocks ND and DQ themselves because they (and everyone around them) thought their loaded rifle had an empty chamber. Lucky neither of them shot someone.
Yeah, it really can be an issue.
I watched a well known and respected shooter (will not name names because he owned the mistake at the time) ND into the parking lot after a match because he thought his mag flag meant that his hot rifle was empty.
While finishing the last stage of the day he had loaded a round but not closed the bolt, then pulled the mag, inserted the mag flag, and carried the loaded rifle down to the parking lot on his shoulder. Once he got to his car he pulled the mag flag, closed the bolt on what he thought was an empty chamber, and fired the rifle, expecting a dry fire and instead firing the live round he didn't know was in the chamber. Luckily for everyone standing around (myself included) he wasn't pointing his "cold" rifle at anyone or at any of our vehicles.
Sure, the shooter made a lot of consecutive errors (like failing to visually inspect the chamber at least twice, assuming his mag block indicated an empty rifle) and also avoided tragedy by following another rule (muzzle awareness, even with an "empty" firearm).
But the point is that the "safety device" so many people in this thread are assuming indicates an cold firearm failed, and it failed because it encourages complacency by allowing guys to pretend it indicated an empty chamber when it does nothing of the sort.
IMO we'd be better to use nothing, assume everyone's gun is hot, and throw people out for poor muzzle discipline at any point during the day than to allow them to use something that actually encourages complacency and unsafe behavior.