Nobody buys a Cadillac Esclade because it is a practical vehicle that meets the needs of the family. It is a status symbol. Unless you and four other members of your immediate family are morbidly obese, and have to commute together on a daily basis, you are kidding your self. I get the guy who drives a beat up pick-up truck to the job site everyday, he might have to haul a ladder or plywood on a weekly basis. But when I see a lifted V8 truck with chrome in the suspension and off road tires that have never seen a mud puddle, there is no justification for that decision besides ego.
There are social norms that become self perpetuating in a society. Often being influenced by marketing and other indirect corporate interests. Often some myth is associated with them like the myth that a larger car is somehow safer. (Anyone with an understanding of physics can explain how a 2500lb car is much safer than a 7500lb SUV). Furthermore, despite fossil fuel industry efforts at discrediting, more and more Americans are becoming aware of climate change and the contributions of the individual.
Buying a status symbol car that is inefficient not only inflates the individual's carbon footprint, but also contributes to the social norm that such a choice is still acceptable. For every single person that buys a gas guzzler this year, they are partly responsible for every gas guzzler bought in their neighborhood next year by sanctioning the social norm. People are social creatures and subject to non-rational drives to conform. If the community they belong to (or strive to belong) sanctions wanton consumption intrinsically, if not explicitly, then they will harbor a strong desire to consume at a level considered normal.
In short, no man is an island. The decisions you make, especially for something as visible as your choice of transportation, has direct effects on the community around you. If you choose the path of wanton consumption over careful consumption of non-renewable resources, not only are you responsible for your own contributions to the global carbon budget, but also for influencing trends in your community. We can no longer afford to cling to the childish fantasy of insulation from our subtle; yet striking in aggregate, influence of our community and environment.
The "if I don't buy it someone else will" justification is a form of cognitive dissonance that is dangerous as a long term trend. If grossly wasteful products sit on the shelves un-purchased, the result will be a clear market trend that the manufacture has to listen to. This is a bottom up change that we collectively have complete control over. Careful resource consumption as a criteria for purchasing decisions is no longer the luxury of elite "environmentalists" but essential for all consumers if we are going to transition our economy to one with longer term goals.