While we shouldn't jump the gun, I would like to pitch in my two cents (or maybe one).
The duel-wielding addition to the game is definitely a huge step, while the direction it is heading towards is equivocal. Personally, I would enjoy this change if it had no relevance to combat. Being able to hold torches and a pickaxe at the same time to facilitate mining, or holding seeds and a hoe to facilitate farming would greatly enhance the survival experience. However, if shields are to be introduced in the game, I'd assume it would be an alternative to blocking. I would find it really unnecessary since, in theory, shields would make it a whole lot harder to kill someone. However, we should investigate exactly how Mojang goes about their implementation, and how they introduce this item into the game. Maybe it won't be as crazy overpowered as we originally thought.
If they manage to successfully integrate shields into PvP, I'd imagine it would be a breath of fresh air. It would add a brand new layer to PvP, dynamically altering it to involve more skill. Not only thinking about the offensive, but also counter-acting your opponents moves. Then there's the argument that these additions may make the whole PvP experience too confusing and complicated for the spring chicken.
In conclusion, we'll have to see how these new feature pan out in the future before speculating. There have been only two snapshots released, and snapshots are almost always highly experimental.