The parrying rules I mean is a different set of those, very close to your attack VS. defense one, actually, and it's to say it works. Thing is, you "roll against the attack" in D&D, when in your case, from what I've gathered, you'd simply roll defense in advance. I didn't grasp it entirely before, so I assumed you'd wait for attack rolls to then roll for defense, but from what I see, you actually give characters a "randomizable" AC, since they roll for defense, and that's their AC for that turn (or whatever measure of time you use). THIS works. Pretty well, actually.
But I'd still advocate to not let too much to the dice, you could merge both systems, and end up with a basic AC (or defense) value, which would be modified for his defensive roll (so a fully-plated warrior would still have a good enough defense on a roll of, say, a 1 on a d6, which makes for a more realistic approach). Damage could follow the same trend, with a basic attack modifier, plus the roll for the turn. And the thing about using HP rather than a wound-type system would be EXACTLY for that reason, lowering the chance of a PC getting killed in a single blow, because believe me, I've been a DM for a long time, and it IS frustrating to players when that happens. So much that some players will quit because of stuff like that. ANd even if the RP is about figuring stuff out, having a good ol' battle IS still one of the most fun parts for a lot of players, and given a battle with, say, a Lord and his 10-14 minions, the amount of dice rolled is almost bound to wipe at least 1-2 guys out of a 4-6 PC party.
My only advice to you, as a DM, is this: if you have to choose between a small lacking in realism and a rule that makes combat more deadly (thus risking the PCs life even more), I'd choose the former. As I said, though, YOU are the DM, so it's entirely up to you what to do. I was just offering some advice so you don't risk pissing of your players, after all, they are the ones that make RP'ing possible after all, and it IS all about them. I'd advise consulting your playgroup as to see what they think about it.
On a sidenote, what I meant was that both our systems deal with a simple rule: higher roll damages lower roll by a figure based on how higher it was. However, since mine is an absolute system, the higher the roll, the higher the damage, until the character drops, while in the system you suggested, when depleted of Light wounds, three rolls of 2 against three rolls of 1 would be enough to outright kill a character, when the blows were ALL poor attacks anyway, while a character with a high toughness (translating into a larger hp) would survive this flurry of attacks with not much more than a scratch, which ends up being more realistic. But I guess it could be tweaked further for allowing damage to specific body areas, but again, as I said, it's bound to hurt PCs more...