Apologies for arriving late. OTOH, I wouldn't have had much time to take part in these thread anyway. But since my name has bene mentionned somewhere, here's in a few words my take on this, that I'm trying to explain in detail in my PhD, and had the opportunity to put forward last year in the Cluny exhibition and the related publications :
Maybe it's not just limited to arms and armour, but one might argue that 3 things dictate the shape (and evolutions) of these object. The choice of words is poor, but let's call them
Technology,
Use and
Fashion. And the 3 of them are entwined, inextricably. I'm separating them to make my point clear now, but please keep this in mind.
Technology is probably the easiest thing to explain : the materials and associated technical processes available at one given time do show on the final objects. Examples for this abound : plate armour (and a lot of polearms) develops when plate-making (in terms of semi-finished products) soars (linkd tot he developement of hydraulic-powered production centers). Earlier, the sandwich construction of Dark Ages sword guards basically dictates their aspect.
Use is also self-explanatory, although maybe too often seen in simplistic ways. But in a few words : the shape of your weapon is dictated by the variables of combat. A simple example is "pointy stiff sword vs. armour (and gaps in it)". But other factors come to play than just passive, defensive parameters. It is tempting, for instance, to link the appearance of fencing terminology in French litterature and the evolution visible in the fighter's equipment of the time. Or in other words : the shields moves closer to the body, the sword gets longer (an moves forward), the cross guard widens as the sword is now opposed not to an active shield primarily, but to an active sword. It might be the beginning of sword-on-sword fencing (albeit limlited) as we know it. But I'm drifting.
Fashion, now. Fashion is the most immaterial influence of these 3, as it is not directly linked to a material, 'hard', physical ensemble of influences. In fact, in that single word I also put all the immaterial aspects that are involved in what can be seen on period arms and armour.
Examples of this have been mentionned before. When something becomes fashionable, then things tend to adapt to these self-created needs.
But it also works the other way. There is a lot -
a lot - of what I call 'inertia' in the evolution of arms and armour. It's not just a matter of 'not fixing what's not broken'. It's deeper than this.
Take a look at pattern-welding, for instance. PW has been explained as a technological need to combine soft iron and hard steel to get the best out of the 2. And that can make sense. Now, what of these swords that have just thin coating of PWed material on the very surface of their flats ? There's no structural benefit to it. And it still remains a PITA to make, and people still made it.
Think of this : it's a bit like the Hogfather in Pratchett's book : if he doesn't finish his round, the Sun might not rise - there would only be a huge ball of fire in the sky. Technically, it would be the same. But they all know it wouldn't be the Sun.
Same for swords : it wouldn't be a sword if it doesn't have the pattern on it.
I could go on for hours on that - or set up an exhibit

- and speak of how the PW elements turned into the prayers inlaid in sword fullers, but that'd be drifting again.
I hope it all makes sense. Sorry for the lengthy speech.
Back on topic now :
Ultimately, I believe that swords are better at what they're made for, with all the things implied by what's above. The design of one sword might have been mostly dictated by efficiency in a specific set of conditions, but it cannot escape the influence of the technology that created it, nor the influence of fashion, be it evolutive or, contrarily, inertial, that surrounded it.
Or in other words : there might be swords that were just designed as being 'good' looking swords, ie satisfying more the immaterial part of this triple set of influences I wrote above, than to be 'good' at doing more material things for which nother sword might perfor better.
Don't get me wrong : I'm happy to see that a lot of people here have this tehcnical, user's approach on swords, especially when compared to the old ways of looking at swords simply as Art objects. We simply shouldn't forget that there's more to the sword than meets the eye (and it hurts).