Well I found an interesting link on this, but was told I'm not to share links to other (apparently especially that one) forums, so I'll try and sum up the parts of it that pertained to your question. I can pm you the link if you want to read more.
Basically, it comes back to stability. Hill style bows are exceptionally stable, and thus can be more forgiving than modern bows. There was some interesting stuff about deflex actually reducing stability by allowing more play at the tips, and an analogy about golf drivers and how they center the weight AROUND the ball to increase stability, rather than putting the weight behind the ball. Not sure how accurate of an analogy it was, but it seemed to make some sense. I'm still of the opinion that a modern bow offers a better balance of performance and stability (when well done of course), but again, it comes down to your shooting style and what bow you're enthusiastic to shoot. My experience with Hill style bows is restricted to one low end bow and maybe 40 shots, and personally I prefer something with a bit more of a hybrid shape, but who knows, I've yet to shoot a real hill!