The thing that you're missing with Quest skill requirements is that you aren't considering the difficulty. Getting 13M in assorted skills for quest requirements is MUCH easier than 13M xp in RC, Agility, or Slayer.
Personally, I think that it depends on the skill. On average, a skill cape is harder to get, and thus more respectable, than a Quest Cape. On average. But when you compare a quest cape to a Cooking/Fletching Cape, then the Quest cape is more respectable. Something like RC or Herblore, a QP Cape can't stand up to that. So IMO, Skill Cape > Quest Cape
I think the reason Dragon Platebody died out so fast (interest wise) is because of the weakness of Dragon Armor in general. On a price:bonus ratio, I'd rather get Torag's. On topic, I think the Pickaxe would come next, seeing as the Axe came out a while ago.
No you shouldn't be able to hide them. It's a source of income for Jagex... for F2P, seeing the advert gives Jagex money (companies pay to have adverts on Jagex sites). For P2P, you pay directly from your pocket.
Cooking is by far the least respectable, followed by Fletching and then Thieving... Theiving is very easy to get by doing Blackjacking and PP, few people realize it. Firemaking is kinda of respected to me, out of the most common skillcapes FM is the most annoying.