My observations, having spent a lot of time with 5,6,and 7 skill:
Ranged forcefield improves a lot with increasing shield skill; 7 skill with an ECS means you never have to worry about archers in front of you if you shield is raised, nor archers to your left even if it isn't raised.
As for melee? I don't think anything affects melee coverage with shields (width, shield skill, etc), everything from a buckler with 3 skill to a huscarl with 10 skill will leave you raging at attacks that inexplicable swing through your block, even when you are directly facing your enemy. The devs have said that shield blocking in melee is exactly the same coverage-wise as manual blocking, but I don't buy it. People manual block me even when I'm totally behind them, but shields often fail to block someone that's exactly in front of you.
I've starting wearing a ton of armor, just because you can't even count on shields to actually block people in melee. They do their job against ranged though.