Our policy, like that of many companies, is not to comment on future plans or work in progress. There are many good reasons that companies as big as Apple and as small as one-person shows adhere to such a policy. One reason is to keep attention focused on what is already available. Another is that keeping your mouth shut about work in progress is a way to implicitly under-promise and over-deliver. When a company says “We plan to ship X in the next three months” and it turns out to take six months, customers are naturally disappointed.
When you say what’s coming next, people naturally want to know when. And when you tell them how long you think it will take, you’re giving them a guess, but to the customer it feels like a promise. And at heart, we’re all optimists about how long our work will take. In short, talking about work in progress and future plans is often a recipe for disappointing your customers.
From: http://daringfireball.net/2013/09/vesper_whats_new_whats_next