If there are so many clinical, professional, and literary examples of doubt being considered undesirable, what could be praiseworthy about doubt? Hindsight being 20-20, you don't need to think too far back for examples. Just think of how the current economic crisis might have been mitigated if some of our business and political leaders had at least a modicum of perspective doubt about the wisdom of their decisions ("no down payment teaser loans," "de-regulating the securities industry," "bank's 30-1 leverage," "credit default swaps"). For them, doubt would have tempered hubris, the pride that went before the fall.
There are two sides to the "doubting" coin. Doubt in the negative sense is associated with people who are nervous, obsessive, and fearful. The doubtful person is often the type who is thought to be "white knuckling" it through life. This is the doubting that is terrified of uncertainty or risk. It's what we might call "amateur doubting."
On the flip side, the astute "professional doubter" is well aware of the finite nature of all "knowledge" and the limitations of all theories and "models." This kind of affirmative doubter, if you will, is calm, unruffled by the inevitable ups and downs of life, and is appropriately skeptical of myopic, absurd claims of certainty and safety ("this is the war to end all wars," "real estate can only go up," "it's the end of history," "there are weapons of mass destruction in Iraq" – we could go on ad infinitum with examples).
This kind of doubter actually is an optimist, one who perceives the silver lining in the dark economic cloud (e.g., the recession actually creates a lot of opportunities), the great book that can be written about social injustices (think Solzenitsyn's "Gulag Archipelago"), and the resilience forthcoming from recovery from a major illness (consider Lance Armstrong).
The affirmative doubter keeps things in perspective. This doubting correlates with a more even temperament and less dichotomous (either-or) thinking. Because they are less reactive, affirmative doubters are more likely to cope well with adversity. They are less prone to catastrophize "the sky is falling" and also less likely to be seduced by claims that "the sky's the limit" (a.k.a. "irrational exuberance").
They know the pitfalls of making prophetic declarations and believe in their infallibility. The affirmative doubter might be less likely to plunge into war without awareness that many of the consequences will be unforeseen, more aware that economic stimuli could backfire, and cognizant that leveraging one's way to wealth is also a way to leverage one's self into bankruptcy. They might be more likely to wait for the risks worth taking rather than taking worthless risks.
Instead of scoffing at doubt, perhaps we would all benefit from some. Not the amateur kind, of course.