The wine glass is designed to hold about 1/3 to 1/2 of a fill to properly release the aromas in the wine, for maximum enjoyment. So to fill wine glass only to the widest part of the glass is the fact that it creates most surface area for the wine. It allows wine to react with oxygen and open up + its easier to swirl around for nosing or just for general examination.
In order to have full aromas from the glass, you have to spin it a little in the bowl so that it oxygenates a little more. When the glass is too full it is difficult to do correctly. Also, good wine glasses have a smaller opening and are closer to an egg shape than a V shape. This helps to concentrate the aromas (and reduce spilling when you swirl the wine).
So, the answer is, it's not really rude to fill your wine glass up. You just look like an uneducated and uncultured moron, or somebody who loves a good time. The drunk is in the eye of the beholder. The wine glass has a widest part where the wine surface would have the largest exposed surface, this allows aromas to evaporate and fill the "empty" part of the glass where you can smell them when drinking it to enhance your experience. Not doing so reduces the experience, like pouring it into a non-transparent glass also hides the colour (also a key part of tasting). Some people might think pouring a full glass shows excessive thirst, laziness and not being measured in your drinking. It is not rude to fill a wine glass more than half way! But it surely would look like an insult (towards the wine, I suppose :) ). Happy drinking everyone.