Jackard wrote:But then, how useful can stockpiles really be for building if you have to constantly pick up their contents and move them somewhere else?
Well, the purpose of drawing materials from stockpiles wasn't so much to make it easier to build things than it currently is in Haven, but rather to make it as easy as it currently is, given that construction sites no longer store materials in themselves, and I do think they do that much. Moving materials to a stockpile is just as easy as is moving materials into a construction site (arguably even easier due to some associated UI changes).
There are aspects of it that does make it easier to build, however, even if that wasn't strictly an intention. For instance, if you want to build something carriable (like a chest or a barrel), you can just plop down their construction sign right next to a stockpile and build it right away, and then carry it off, which I have found a nice thing.
Jackard wrote:Can you put materials in a wagon or minecart or something to act as mobile stockpiles?
There is no such thing currently, but we might consider it as it sounds like a potentially nice thing. :)
As for something that reminds slightly of it, however, it is possible to hold both a construction site and a container open at the same time, and thus to use the contents of that container directly for building without having to transfer it to your character's inventory (similarly to how it is possible to craft directly to and from containers).