Edit: To clarify, when I talk about "setting expectations," I mean strictly from a design perspective. In an actual game, combat takes as long as it takes, and if that means the whole fight is decided in one round, so be it. But for a game designer, trying to figure out how much damage a given weapon or spell should do, it's essential to have a baseline of how long a typical fight is expected to run.
No, it's not. In fact, a lot of games don't use that approach at all. Which is what we'd call a "gamist" approach - build the reality of the game world around what works for the game. A lot of games (I'd say most of them) instead go for a simulationist approach - what makes sense in the "real world" they're trying to portray (whether it be Medieval Fantasy or Shadowrunning Orks, or whatever else).
So, instead, the designer goes "This is a longsword. It'll do... um.... 1d8 damage. A dagger does less damage than a longsword, so we'll make it, um... 1d4. A mace? Um, it can hurt, but only if it hits you right, and your armour will deflect some of the blow, so, uh.... 1d6. And a claymore is like a super big longsword, so we'll say it does 1d12." And so on, and so forth.
"Round Length' seems like a very odd thing to design a combat around, unless you're playing 4e, in which it was a primary factor. (And I don't like it, personally - how many one round fights have YOU had in 4e? I haven't had even one, even if the PCs have all the advantages). To me, I'd say build rules that simulate your idea of what "reality" should be in the game world, and then start extrapolating from there:
"Okay, with these combat rules, the average combat is around six rounds long. I imagine fights in my game world would last around two or three minutes, like a good movie. We'll say two and a half minutes. Uh. I guess I can get away with saying a single round is thirty seconds long. Okay, cool!"