Having been here for almost seven years, I've concluded that there are as many SketchUp workflows as there are SketchUp users. Trying to recommend a single catch-all combination of tools and methods is like stapling Jell-O to a piece of paper — difficult, messy, and ill-advised. When tools like DoubleCAD XT (and DoubleCAD XT Pro) from IMSI/Design come along, things get a little easier to explain.
Most SketchUp modelers who design things for a living spend their time somewhere between the second and third dimensions. Models are 3D, but construction (or fabrication) requires 2D documents. Often, those 2D drawings are too complex to use LayOut in SketchUp Pro to make; LayOut isn't a dedicated drafting tool, after all.
DoubleCAD XT is (as its name implies) most certainly a full-featured CAD tool. It's more affordable than AutoCAD LT, and it "holds hands" with SketchUp better than any other CAD system on the market. Among other things, DoubleCAD XT:
- exports COLLADA files which you can open in SketchUp
- imports .SKP files natively
- imports and recognizes SketchUp layers
- generates viewports from scenes in your models
- converts components into DoubleCAD blocks automatically
If you're a Windows user who routinely creates 2D documents, you owe it to yourself to give DoubleCAD a whirl. The no-cost version (did I forget to mention that DoubleCAD XT is free?) is extremely capable, and the Pro version adds lots, lots more for US$695.