Well, remember that each round is events over several seconds. It's not a blow by blow (even though I've seen a lot of tables treat it as such). There could be ongoing swaps between favoring the shield to block some blow then adjusting your grip to counterstrike. There are even some non-European shields that employ a bracer or half-glove apparel to allow freedom for the hand (and, as always, fantasy versions to boot).
As an aside, I use multiple shield types in my own game (bucker is +1 AC, round is +2 AC, kite is +3 AC, tower is cover). I'd like to do bracers (without them having to be bracers of defense) as well, but haven't really thought out a system for them.