[ewanmcteagle]

You must ask this question of every single piece of stock advice ever given freely or sold. If you knew about future price movements with some degree of accuracy, the least profitable thing you could do would be to tell others.

[kurumo]

That is not entirely correct. If you have already acquired credibility (somehow, doesn't matter how for the purposes of this exercise), your estimate or advice can (and frequently does) become a self-fulfilling prophecy. Knowing that a significant number of investors will follow your advice gives you information which can be used in the market. It then stands to reason that the most profitable thing you could do would be to give this advice to the maximum possible number of people.

[extension]

No particular reason the advice has to be accurate.

[sliverstorm]

accurate advice is more likely to come true, thus bolstering your credibility.

[extension]

But the advice comes true because you gave it. This is the inherent flaw with the market at issue here -- it selects for consistency, but not accuracy per se.