Digital modeling is not the same as sketching... that would be more like building small mock-up foam or cardboard models. 2D Sketching can be done in many software programs... including photoshop, paint or any other drawing software, however only a few CAD programs include a freehand sketching module. The advantage of studio tools is that you can migrate from 2D crayon marker sketching to modeling directly on the same program. That's why studio tools was the winner on that particular research study.
We've come a long way since the first CAD programs... and the advantage of modeling WYSIWYG (what you see is what you get) instead of the boring coordinates command lines in old cad programs. But I admit that the old drawing techniques are still quite useful... digitizing a "napkin" sketch and then turn it into a full rendered presentation sketch in minutes is something that I encourage from my students. But it also works the other way, I just love when they use the wireframe renders and colour them using traditional markers... the accuracy of the digital model, plus a good markers technique can make a sketch look amazing.
So, here are some tutorials I've found about sketching ont the product design forums: Mikael Lugnegard from Umea gives very good tips on how to hybrid-sketch, this screen captured movie of a chair sketch using sketchbook pro and other illustrations by Waikit Chung (thanks for the updated URLs), and this jpg sketch tutorial by Renzsu for wacom tablet users. Cool!