Why now is not the time for Democratic candidates to be getting specific on policy

In the early days of Hillary Clinton's 2016 campaign, it was supposed to be this great thing that she was going on a long "listening" tour to hear from us, rather than starting out by telling us exactly what needs to be done. Now, I'm seeing a lot of Facebook posts saying Elizabeth Warren is the most impressive candidate because she's released a large number of specific policies very early on. I don't find that impressive, and I've never heard of this idea that the candidates should race to publish as many policy specifics as early as possible in the primaries. Also, if starting your campaign with a focus on listening to voters is such a good idea, couldn't it be better to hold off on getting too specific about your policy proposals at the very early stages of the primaries? Shouldn't the candidates be open to the possibility that listening to voters could change what policies they want to propose?