I disagree; I think the "Castle in the Sky" dub is pretty fantastic and watchable on its own right, even if the line change of Sheeta's speech at the end is out of place. Still, I really liked it a lot and I honestly don't find much wrong with it. Yeah, the leads sound like teenagers and the script sometimes too chatty in places but the enthusiasm the actors show for their characters and the gorgeous music rescored by Joe Hisaishi more than made up for those two faults. Mark Hamill is not the only good thing about the "Castle in the Sky" dub either; Cloris Leachman is also excellent as Dola, the film's other interesting character. I can't imagine anyone else but her doing that role. The supporting cast was also quite entertaining; it's not often you get a dub where Jim Cummings, Mandy Patinkin, Andy Dick, Mike McShane, Tress MacNeille, and Corey Burton are all in the same show. It may not be the most accurate of the Disney Ghibli dubs and is not perfect, but it is in no way abysmal. Taken on its own, it's a brilliant listen in its own right once you get over its faults.
I personally say the worst Miyazaki dub is the older 80s JAL version of "Castle in the Sky"—it's more accurate than the Disney dub, true, but its actors sound practically lifeless and detached from their roles. Even if Pazu and Sheeta sound younger, I found their actresses in the older dub to be much lamer. James van der Beek and Anna Paquin are not perfect by any means, but compared to their '80s counterparts, they are nowhere nearly as wooden or lifeless. Yes, there is the occasional missed line and the off-again-on-again accent issue of Paquin, but their performances sounded more natural to me than the '80s dub. Then there's "Warriors of the Wind"; which basically turned "Nausicaa" into a 90-minute kidfest. Disney's dub of CASTLE IN THE SKY may have more dialogue and music, but at least it doesn't cut anything from the film and is certainly far better than those two.
Don't get me wrong I like the Japanese version, too, but Disney's dub also has its charms. Mark Hamill is one of them. Yes, it has its weaknesses, but that doesn't mean it's worse.