This is an interesting question.

I'd draw a distinction between "motives" and "ulterior motives" intended to decieve or mislead. Both types of motives occur.

As a quick segmentation analysis, we could group commentators on oil issues roughly into political groups, corporate groups, and intellectual groups. The categories are not mutually exclusive. Other groupings do, of couse, make sense.

Political bodies include the IEA, EIA, USGS, and OPEC. They often have to support a homogenized point of view that is politically expedient. Many inside the groups may not really believe what they say. Consider the outrageously high USGS estimates, but the fact that they also publicize Les Magoon's peak oil viewpoint. I feel a lot of these groups are either disingenuous or have clear ulterior motives.

Corporate groups are the oil companies and other associated producers and users. A very few choose to speak openly (Chevron's "Will you join us" campaign). Some speak in code (Exxon Mobil's graph showing non-OPEC supply flat within the decade). Most don't speak, for good reason. Saying that your industry has no long-term future in its current form is bad for stock prices, and could even lead to lawsuits. I think these corporates will tell the truth when they believe it's good for business, and be discreet and quiet when they think the truth causes more trouble than it is worth. I don't think there is much outright lying here, perhaps because of liability issues as much as anything else.

Intellectual groups are those people studying the issue from a more outside perspective. I suspect most of these people are sincere and motivated by the truth as they see it. There are diverse opinions in this group: I'd put the Lynch & Yergin cornucopian / economic camp in this groups; as well as Hubbertarians/ depletionists like Deffeyes, Campbell, Simmons; environment policy people like Heinberg; social critics like Kunstler; and the more extreme post-industrial and dieoff groups. Some have clear agendas, such as pushing environmental or climate change policies, or debunking peak oil. But that doesn't mean they are lying--it's how they see the truth.

Dave said in an earlier post that peak oil is not a matter of belief, but I disagree with that. I am a fervent peak oil believer, but I can't (yet) prove my belief to a smart skeptic. The data is simply too bad, too incomplete, too full of holes. Ultimately, we don't know people's motives, but we can judge their words and actions. I'd assume sincerity, even from those I vehemently disagree with, unless we have clear evidence to the contrary.

I recall listening to a BBC interview in the 3rd or 4th week of May 1990 (~2 months before Saddam Hussein invaded Kuwait).  They were interviewing the VP for Marketing of the weapons division of Vickers, a British weapons manufacturer.  At some point during the interview the interviewer asked him if Vicker was going to de-emphasize the weapons side of their business now that the Cold War was over.  He replied, "Oh no.  We think there is still a large market for weapons, especially in the Persian Gulf."

I offer this and subsequent events as evidence that the worldwide military-industrial complex considers oil as crucial to their marketing strategy.  If oil is becoming scarce I fully expect that it will lead to a windfall for them.

Thanks for this question about motivation.  I have thought about the issue more from the perspective of trying to understand what unconscious motives or subconcious needs might cause us to deceive ourselves in these questions that have great impact in how we decide to live our lives.  Cuz when you decide to junk your cushy pleasant life and buy the organic farm in Sasketchewan, you don't want to realize later that you allowed yourself to be mislead by someone with mixed motives.

We in the Peak Oil camp like to point out that our spokespeople are more believable than governments or corporate spokespeople because our champions have no vested interests in the outcome.  They are retired, they are outside the system, etc.

I beg to differ.  At a very basic level, we have a need or desire to be proven right.  We tend to become attached to an argument and world view, and an unconscious need to be proven right can become a powerful motivator, especially for those who define themselves as intellectuals, as ones who have tasted of the tree of knowledge, and who can discern not only what is true from what is false, but what the future holds, and how the rest of humanity must conform to their divine revelation or perish.  This motivation can skew our interpretation of reality, causing us to filter incoming data that interferes with our world view.  I humbly suggest that this motivation runs deep and has a long history.

Along with this subconscious need is a tendency for groups to begin to reinforce each others opinions, as noted here:
http://www.energybulletin.net/10113.html, where the author mentions the study on firemen adopting similar viewpoints in their close knit firehouse societies, viewpoints seen by any outsider as silly.  So this is not so much a motivation but a group behaviour.  In public policy debates, this group think tendency leads us to polarized, inflexible group positions, and an inability to thoughtfully consider other viewpoints.  

When I read the Time point-counterpoint essay, I was struck by the degree to which this is already a polarized issue in which each camp trots out the same verbiage and arguments, replete with key emotional trigger words which quickly identify one's faith allegiance and remove the need for any further thought.

I am glad though that we can agree here to leave religion out of it and resolve everything through our pristine scientific objectivity.