Right now it's kind of backwards: the repetitive rooms like bedrooms we have to assemble piece by piece, even though the placement doesn't really matter - the workshops, of which we normally have just one or two in a whole map, are only available in ready-made format.
Room-like workshops would also allow us to customize the size (eg. a butcher's room with four chopping blocks and four cleavers) and functions (eg. an anvil without furnace can be used for cold hammering (mint, metalcrafting), but not for smithing).
We'll also have the ability to improve our workshops later on if the need becomes clear, giving more room to evolve and build. For example, if we put lots of storage items in a room, that will reduce clutter. With some flexibility in the definition of the room, we could improve efficiency by careful placement (for example a great hall next to the pastures, with the butchery as a central point and all derived industries next to it - with only the tools they need to process that specific resource).
(On a slight tangent, that would imply that every room had a list of tasks. Green: tools present, task allowed. Gray: tools not present, task not allowed. Red: task allowed, tools not present. Blue: tools present, task not allowed. That way you could ensure that jobs weren't done at a random table in a random workshop, because many jobs would just require a table as essential furniture.)