This is actually a quite controversial case, though I admit that the label Belgian is a little misleading, "Trappist Quadrupel" is not so bad. Stan Heironymous, in his very well researched book on Belgian Brewing "Brew Like A Monk", writes about La Trappe, which was briefly a trappist brewery in the Netherlands and had the appellation revoked when it sold to a Dutch Brewing company. According to those that sold, the problem was simply maintaining an order of monks. They did produce a beer which they called a Quadrupel. Moreover, Quadrupel has become a label on some american beers obviously of Belgian inspiration (and using derivatives of trappist yeast.) While this arguably makes Quadrupel not a style, Quadrupel is not listed as a style per say, but one of the myriad "specialized" belgian ales.

 I really find your argument unconvincing, since the style guidelines make it clear that this category is to be used for submitting to beer competitions with a beer that is not appropriately labeled as something else, which is to say that the style category is quite upfront about what it is and not misleading at all. Yes, the BJCP styles include some catch all categories that aren't rightly considered styles, but they are clear about it when a judging category doesn't not fit some historically entrenched approach to beer. Since the guidelines are very clear about what they are but also provide a lot of accurate information to the reader, I think they're great. (Indeed, I would appriciate it if those that object would show me where the BJCP contradicts the opinions of other groups on these subjects.)