It is generally seen as important and independent enough to be equivalent to a Kingdom, but they never actually claimed the title.
The semantic disconnect here is that the entity the Valois had by this point was a de facto Kingdom of Burgundy. They were "Dukes of Burgundy" in the same way that William of Toulouse during the 1st Crusade was the "Count of Toulouse" even though his actual power was greater than most Dukes - he was one of the richest and most powerful lords on the whole trip. That didn't mean that the County of Toulouse had changed: it just meant that the guy ruling it also controlled a huge other area next door.
There were several good reasons to do this. It let people with higher ranks feel less threatened by you because they could tell themselves that they were superior even if you had more power.
The Valois did not re-draw the borders of the Duchy of Burgundy - they possessed a Burgundian state that probably would've lasted longer had not Duke Charles wanted to make himself a King. Before them and after them, the Duchy of Burgundy was pretty much the same, the big exception being the pre-Medieval borders which did change quite a lot:
6th century Burgundian Kingdom:
http://en.wikipedia.org/wiki/File:Map_Burgundian_Kingdom_2_EN.png9th/10th century Burgundy - notice the County of Burgundy is the German portion whereas the future "Duchy of Burgundy" doesn't even include the county:
http://en.wikipedia.org/wiki/File:Karte_Hoch_und_Niederburgund_EN.pngMedieval Burgundy -
http://en.wikipedia.org/wiki/File:Map_Kingdom_Arelat_EN.pngValois Burgundian State -
http://en.wikipedia.org/wiki/File:Karte_Haus_Burgund_4_EN.pngBurgundy is really confusing because it could mean several different things over nearly 900 years. What you're suggesting is also illustrated here pretty well in that you had divisions between French Burgundy and German Burgundy that are also still around today and those divisions came about because you had some Dukes of Burgundy who were either allied to or subject to (if not a direct part of) the French Crown while you had Upper and Lower Burgundy that were more German and part of HRE politics. This type of confusion is what you get when you take what had been one cultural and political entity and try to re-draw it: a big ugly mess for hundreds of years. But the re-drawing happened on a level above what I'd consider the Duchy of Burgundy. Duke of Burgundy was a very prestigious title, so it's not like you're taking a bit hit by doubling or tripling your territory and still having that be your primary mode of address.
Another analogy would be the Prince of Wales. As you pointed out, they are usually also the Dukes of Cornwall and of course heirs to the English Crown. But that doesn't change the cultural or political borders of Wales. It changes the domain of the guy who happens to rule in Wales.
Real changes in Duke-tier fiefdoms were pretty uncommon precisely because they were cultural as much as geographical. It makes sense, too, because even if you went around poaching land from other nobles, you really needed legitimacy. It wasn't enough for Edward III to beat France on the field and have total military control of most of the country: he needed to make the argument that he was the rightful heir due to some 700-year-old Frankish law that excluded women from the line of inheritance. He had to make the case that he wasn't some English guy coming in and taking over: he'd actually been French all along, and not just that, but more French than the other guy!
Before you had censuses or lots of highways or standing armies, how would you even draw borders? You'd go around finding groups of people who talked and acted more or less the same and bunch them together under some name. If your boss was strong enough and there weren't many other power centers you could get away with one name for a huge area (King of the Franks!) but once all that fell apart and you actually had local economies rather than barbarians running around in the woods, suddenly Burgundian Franks were different enough from Savoyard Franks or Orleans Franks that anybody who spent enough time traveling around could start drawing lines between one group and the next. Military conquest might re-draw those lines a lot but if you look at all the counties in France and Europe in 1187 or 1289 their names will sound familiar because the basic cultural entities are still there.
Even in 2012, Connecticut could invade Rhode Island and get away with it long before Anjou could invade Brittany and get away with it.
