Until his shield breaks or he fights someone with crushthrough. Having a shield also makes a 1 hander VERY slow, which is a good tradeoff. If a 1 hander puts away their shield, they are faster but they still have shorter range and do less damage than a 2 hander. Right now, I think things are pretty well balanced except the shields themselves could use some variation. As many have said, huscarl is the smart choice unless you pick a shield for looks.
2 Handers have longer range, are faster and do more damage. 1 Handers are slower, have shorter range but have good protection against multiple attackers. I don't see what the problem is. If you're getting spammed by a 1 hander, either his shield is away or you are WAY lower level.
Well, depending on your 1h weapon, you can still outspam someone with crushthrough. And it hardly makes you VERY slow, just slower. The fastest/best 1h swords are still frigging fast even with with a shield. Less damage is arguable, steel pick for example does fuckloads of damage, 1-2 hitting everyone. I'm not getting spammed by 1handers, though, so not sure why you mention it.
I play pure 2hander because I like the added challenge of manual blocking, but if I went for pure efficiency, atm I'd go steelpick+shield&throwing.