Various business stakeholders ask me on a daily basis about our product capabilities, cost estimations, time frames, and many other things they need to know in order to do their jobs. While all these stakeholders expect me to provide them with factually correct information, they nonetheless expect me to foremost help them with their cause and only secondarily to be 100% correct. As Boštjan said yesterday, it's very easy to be 100% factually correct and kill all opportunities, but it's much harder to be factually correct while still progressing business cause.
The role of an expert in a company is not to cover his or her ass by being 100% correct in his or her guidance, but to correct the notions of what's easy and what's difficult as held by his or her non-expert colleagues. Where opinions of non-experts match reality, the help of an expert is unnecessary. But if non-experts consider something easy or cheap while in reality it's difficult and expensive (and vice versa, of course), the role of an expert is essential in order to steer the company away from bad decisions or missing great opportunities. Unfortunately, very few experts understand their role in decision making and instead take a self-righteous pose that's to nobody's benefit.