I think a character with a scimitar in one hand and a longsword in the other could, indeed, by the PH rules, attack with either one. He just gains no benefit, even if he has TWF/TWD and can't use a ranger two-weapon attack power unless he has the ranger use-two-big-weapons class feature.
If you're using a one-handed and an off-hand weapon, you can benefit from TWF/TWD, and use a ranger two-weapon attack even if you're an archer-ranger or multiclassed to ranger, and lack the class feature.
That said, it's still not flavor only. If you're using a light shield, you can hold, but not use, a weapon in your shield hand. You have to swap it to your other hand to use it. If that takes an action, that matters, because it makes the light shield less desireable.
I'm sure it matters for other, equally obscure, reasons, too.