You can't dodge missiles in UrW. A shield is a static defense covering a part of the body according to how it's held, and protects that part against missiles (unprotected body parts can still be hit even if the attacker aimed for, but didn't quite manage to hit, a protected body part, or hit the shield while aiming for an unprotected body part, as the partial miss resulted in an attack against a protected body part).
Melee is a completely different issue from missile defense: I don't think a shield provides any passive advantage at all, but rather has to be used actively to block to be effective (but that doesn't rule out enemies [or companions, if you take the risk of equipping them with missile weapons] attacking you with missiles while you're engaged in melee).
Also, using a shield means two handed weapons can't be used as intended.
I don't know if there's any difference between using a shield to block versus dodging in melee (a shield can be damaged, as can weapons used to block attacks). In both cases the defensive skill can completely negate the attack, downgrade its severity, fail, or fail critically.
Maybe there's someone with a better understanding of the mechanics involved?