I find this compelling...
"The only serious privilege he wields is over the Duchy of Cornwall, where his special pleading to Whitehall is no different from that of any landowner. He runs a decent art college and a worthy foundation, which espouse his range of interests. But this hardly endangers the stability of the state. Indeed, the running message of the black spider letters is not potency but a plaintive sigh of woe at a world going to the dogs. The causes long known as dear to the prince’s heart are organic farming, alternative medicine, opposing GM foods, global warming and traditional architecture. There are strong opinions but no political partisanship in his interventions. The one topic on which he is said to exert unfair influence is over modern architecture" Simon Jenkins (see previous for link).
I know the worry is let him have an inch and he will take a mile in politics, but I could argue what Prince George wears, that Harry got an Aids test and that the family supports English teams more than UK/commonwealth teams has more social influence than the spider memos ever could.