One problem still unsolved by MCAD vendors is how to get initial concepts to a full assembly.
If you are not working with an initial concept, then you are working with existing parts. If the existing parts have no drawings, then you need to convert them into drawings, somehow. There are a variety of methods, such as measuring distances and entering them into a CAD package.
Another method is to scan the parts with a 3D laser scanner, creating a file of 3D points. Software converts the points into surfaces, which can be edited into 3D solid models. The models are imported into an MCAD system, cleaned up further, and then assemblies and drawings are created. While the system is not perfect, software is getting better at recognizing features in amongs the billions of scanned points.
Geomagic announces today in Europe new software that handles everything, from controlling the 3D scanner to outputting assemblies and drawings. Spark is the name, and it combines Geomagic's existing software (point data editing) with SpaceClaim (3D modeling).
In the figure below, I combined two screen grabs provided by Geomagic, showing the scan data converted by Geomagic (blue) and 2D drawings generated from 3D parts by SpaceClaim (green). Because the result is a solid model, all the other tasks are available, such as dimensioning, rendering, and analysis.