We don't have to compare it to proficient fighters
Anyway let's take an example, I trained my character to become really good with dual wielding swords, however while fighting, my opponent manage to disarm one of my swords, and since I don't have any extra weapon on me, I think I'll rather hold my last sword with two hands than one, since I'll get a better grip and more control.
And it's just fun to have to option to choose what you prefer ^^
I'm not sure if holding a single hand grip sword gives you more control or grip, because you are basically putting one hand on top of the other. I think you rather lose control by doing that, because having the other hand free allows you to counter balance the other arm if you try to swing the sword really fast.
Do not forget, that real sword fighting has alot to do with movement coming from the wrist, you rortate and tilt the blade all the time and lots of the force comes from the momentum (mass + movement = mass multiplied). Stabbing hard rather comes from the shoulder.
I mentioned both hand use actually only in relation with special moves, such as executing an opponen that is kneeling infront of you because you brought him down, so you raise the sword overhead and strike as hard as you can by using both hands. In actual combat, where you strike forth and retreat, this would be rather useless I think.
Also, I would say that even two-handed weapons should be able to be wielded in one hand, however since the weight of it, you would not be able to hold it straight up, rather you would drag the sword around and have to use your whole body in order to swing it. Take Fable(1) as an example, even if you didn't have enough strength to wield a weapon properly, you could still "wield" it, however you would drag the sword after you, and as I said you would have to use your whole body in order to even swing it.
Think that would be awesome.
Interesting thing is, that European long swords have a thinner blade and are generally lighter than the Japanese sword, which due to it's massive blade thickness is heavier and is really meant to be used both handed, while a European long sword is technically usable one handed, though you get much more out of it using it both handed.
Do not forget that a long, two haded sword relies on continuous motions, like swinging and rotating and you can only do this from the wrists, which why using a sword both handed does not limit the flexibility and if the grip is long enough the other hand does not only support the weight, but also allows to increase the speed rotation (like when chopping), it's effortless. That increase in rotation would not be given if your sword has only a short, one handed grip and you just wrap the one hand around the other.
A real zweihänder (huge long heavy sword) would be probably unusable with only one hand. A longsword, or also known as bastard sword, is actually a slightly elongated sword, so it's light enough but has a long grip.
I can imagine when fighting a drunk guy, the character could deliberately hold the long sword with one hand to show that he does not need to fear and isn't serious about it.
But still, wouldn't it be more efficient to hold that hammer with two hands if you got nothing else to hold in you of-hand? Even if you're a skilled Swords/Mace/hammer (etc) fighter?
Also about that a skilled sword fighter wouldn't be a total noob with a hammer/mace, you're absolutely right, but i think this wont be a problem, since you train your character in Light or Heavy weapons, and Light weapons probably include all one handed weapons
As I say, basically it depends on the design of the weapon. If it is a small hammer, no point in using it both handed. If it is a larger hamemr, heavier with a longer shaft, you not only need to use hands because of the weight, but also can do it because both hands fit on it. It all about the circumstances, design defines usage.
I don't mean to sound like I know it all, so I apologize for that attitude, it's just the information I recall on the subject matter. Someone who knows more could prove me wrong though. It's just that while options are cool, things should be used the way they make sense and the character would know what the right thing to do is based on his state of skill (noob knows less than a pro). I really don't mind options though, they just need to fit in.
Edit: Found a sword fighting vid, can't watch it from this machine though, so I hope it gives an idea: