I agree with you Atohms, I love a good computer rendering, but after the concept work is done. Nothing beats a sketch for speed of communication, perhaps speech, but that cannot often describe what a few lines on the nearest piece of paper will. Quick prototype in any material, even better.
Why a sketch over a render? A couple of reasons.
A sketch has a openess of interpretation, the designer can look at it later and see something new, the viewer can interpret the sketch in his own frame. Kind of like reading a book or watching a movie made from the same book. The sketch is the book, materials, surfaces, functions, etc. can be imagined, personally interpreted. The computer rendering is the movie, clear and spelled out and fixed in the imagination. At the early stage, what is needed is to paint in as broad strokes as possible, to find the possibilities and find the resonance with the product.
Fear of commitment. When sketching, each iteration for me is the focus. I am usually totally into that design, thinking it is the best one yet. Then after flipping the paper over and starting on the next, it then becomes the prime expression of the idea, I am committed for that moment, to that sketch and concept. This goes on for 10-15 sketches evolutions of a concept until a little stack of paper sits on the desk showing the evolution. 3-5 get sorted into the reject pile, and the rest get kept to think about. Each period of commitment was maybe 5-40 minutes. When I fall out of love with a design, the mourning period is brief, I did not have that much invested in it. When the client does not find the love in the sketches, a new set can be explored and gray ideas refined into new concepts. When I can create a CAD concept in the same period of time, I often do, to print and use as a basis to hand sketch over. However, when a period of time is invested in CAD creation that becomes significant, hours or days to reach a concept, the fluidity of change is lost. I have a higher degree of commitment, and I have spent the time following one solution when many more solutions may have been explored.
Example: ( this could be an actual design summit )
Suppose a company with no limit on resources or budget, let’s say British Petroleum, was faced with a significant physical problem to which it did not have a ready solution. A sticky dirty mess to clean up from a huge area. If it were to fly the best designers in the world to its headquarters and discuss possible solutions, what method would be most efficient? Money is no really really no object in this project.
A whiteboard, pen and paper are going to be indispensable. The room is going to be filled with sketches and the engineers and oil rig workers are going to be giving opinions on what will and won’t work. There might be some pressure sensitive tablets and styluses sketching away, but my money would be on the human hand gripping marking instruments. Hundreds of solutions will emerge from the discussions in that room. People from all areas will be inspired from looking at scratchy lines on paper and thick black lines drawn on washable whiteboards. Hopefully some good ideas would be generated.
Will there be a couple guys sitting at SolidWorks stations parametrically programming and rendering up new solutions? No.
The biggest possible enhancement to have sitting in this room would be a physical scale model of the broken system and a tank full of seawater and crude oil. Pictures on a flatscreen may keep your hands from getting dirty, but they are not going to solve the problem.
To answer the main nebulous question, for an average project, I sketch about 20 initial concepts, follow those up with 15-20 additonal, and maybe 5-10 for each round after that as things get tighter.