"Disclosure of the correspondence could damage the PofW's ability to perform his duties when he becomes king

[BackOnceAgainWithLoopyLoops]

Well, in that case, even more reason for them to be made public. AIBU to think that, if he needs his correspondence hidden as they reveal him as incapable of performing his duties as king, then he should never be king? here

[CogitoErgoSometimes]

YABU.... We require monarchs to be officially disinterested in politics. We don't know what the Queen thinks about anything (unless Frank Gardner tells us) because it's all kept strictly private between her, the PM and others. Prince Charles is also constitutionally required to have no opinion on anything and needs a similar outlet.

[SmellyFartado]

Agree that they should be made public. I think the worry is not Charles being proved incapable, rather, the letters would reveal political leanings or in his lobbying undue pressure on certain matters.

[BackOnceAgainWithLoopyLoops]

Exactly. So why AIBU?

[MainlyMaynie]

They don't say the letters reveal him to be incapable of of performing his duties as king, they say revealing the letters would compromise his ability to perform his duties as king. Which is a completely different issue and a common phrase in FOI. It probably means that their revelation would prevent him offering free and frank views in the manner expected of a sovereign. I think that's fair enough, though tbh the speculation may be as damaging as their release.