This is why I went with sashes (English Civil War, anyone?), although I do need to do more with flowers and other natural forms of demarcation in future.

I suspect part of this use of uniforms comes not just from transposing the modern (and established tropes) back on the medieval, but also taking perceptions of the ancient and moving them forward. A number of people who put their soldiers in uniform might very well point back to the Roman legions (and the popular perception of them) for inspiration/justification as much as they would point forward to later traditions. If the Romans had them, why can't the troops of the Duchy of Gildedepaulettes??? Mind, there are a whole host of answers to this, including things like context and history and rises and falls, but most people don't bother with that. Then again, I've read about enough ten pound long swords in fantasy to not have much of a bar when it comes to expectations and history anyhow (sadly).